For a Mailster SMTP server, besides having a correct SPF record authorizing your mail server IPs, you should also consider:
- SPF Record
- Authorize all IPs your Mailster SMTP server uses to send mail.
- Include any third-party services if used (like Amazon SES or Sendiio SMTP).
- Example format:
v=spf1 ip4:<your_mailster_smtp_ip> include:amazonses.com include:spf.sendiiosmtp.com -all - DKIM (DomainKeys Identified Mail)
- Set up DKIM for your domain. Mailster or your mail provider should generate DKIM keys to publish as DNS TXT records.
- This cryptographically signs your outgoing emails and improves deliverability.
- DMARC (Domain-based Message Authentication, Reporting & Conformance)
- A DMARC policy tells receiving servers to enforce SPF/DKIM and send reports on failures.
- Add a DMARC TXT record with policy like
p=quarantineorp=rejectto protect your domain from spoofing.
- MX and A Records
- Make sure your MX records point to the correct mail server hostname.
- Ensure reverse DNS (PTR) records are set for your mail server IPs (matching IP → hostname).
- Bounce Handling
- Configure your bounce server (POP3/IMAP) correctly and ensure Mailster can connect to it without errors.
These combined DNS records and configurations greatly reduce the chances your emails are marked as spam or rejected. SPF alone is necessary but insufficient without DKIM and DMARC for good deliverability.
If Mailster provides any specific DNS records or public keys for DKIM, use those exactly as given.
If your Mailster SMTP server sends mail using the mail server IP already included in your SPF record, then you do not need to add any additional IPs specifically for Mailster in your SPF record.
The existing SPF record authorizing your mail server IP (the server Mailster uses) is sufficient for SPF authentication for emails sent through Mailster.
However, make sure:
- The SPF record explicitly includes the IP(s) or hostname(s) of all mail servers you use to send from your domain.
- Any third-party services used alongside Mailster (like Amazon SES or others) are included with
include:mechanisms if applicable. - You also configure DKIM and DMARC records for full email authentication and deliverability.
Using a subdomain like mail.domain.com instead of just the root domain (domain.com) for your mail server is generally considered best practice for several reasons:
Reasons to Use a Mail Subdomain
- Reputation Isolation
Using a subdomain for email sending (e.g., mail.domain.com or smtp.domain.com) separates your email sending reputation from your main website domain.
This way, if the mail server or marketing emails get flagged for spam or blacklisted, your main website domain’s reputation stays safer and unaffected. - Easier Management
Subdomains allow better organization and management of different email streams, such as transactional emails on one subdomain and marketing emails on another.
This helps with monitoring and improving deliverability for each stream independently. - Clear DNS Configuration
It’s clearer and less error-prone to configure SPF, DKIM, DMARC, MX, and other DNS records specifically for the mail subdomain without mixing with website or other services. - Industry and ISP Expectations
Major mailbox providers and email sending services expect or recommend using dedicated subdomains for mail sending to improve trust and deliverability.
Subdomains also make it easier to implement domain alignment policies in DMARC. - Avoid Conflicts
Using the root domain as the mail server can cause conflicts with website hosting, complicate SSL/TLS certificate setups, and mix traffic types on one domain.
Summary from Industry Guidance
- Set a dedicated, descriptive subdomain such as
mail.domain.comorsmtp.domain.com. - Authenticate this subdomain with proper SPF, DKIM, and DMARC records.
- Warm up the subdomain gradually if doing volume emailing to build reputation.
- Separate different email types or services by subdomain to protect reputation.
This approach maximizes deliverability, minimizes risk to your core brand domain, and aids troubleshooting.
The main practical purpose of using a subdomain like mail.domain.com for your mail server name is so that you can set a matching reverse DNS (PTR) record on the IP address of your mail server.
Why is this important?
- The reverse DNS (rDNS) lookup occurs when recipient email servers check what domain name corresponds to the sending IP address.
- The PTR record maps the mail server IP back to the mail.domain.com hostname.
- This alignment between forward DNS (A record for mail.domain.com to IP) and reverse DNS (PTR record for IP to mail.domain.com) helps receiving servers trust your email server as legitimate.
- Without a properly configured PTR record, many mail servers will flag or reject your emails as spam or suspicious, harming deliverability.
- rDNS is a key anti-spam and authentication step alongside SPF, DKIM, and DMARC.
Summary
Using a dedicated mail subdomain lets you clearly identify your mail sending server and set a matching PTR record on its IP, improving email validation and deliverability. The PTR record for your mail server’s IP should point to this subdomain.
This is why best practice includes setting up a mail subdomain plus PTR record on the IP, ensuring full forward and reverse DNS alignment.
Reverse DNS (rDNS) is not only important for email deliverability; it serves several purposes, but its critical role in email is especially well-recognized:
Main Purposes of rDNS
- Email Deliverability and Trust
- rDNS lets receiving mail servers verify that the sending IP address maps back to a valid hostname, helping confirm legitimacy.
- Mail servers often reject or flag emails without a proper rDNS (PTR) record as spam or suspicious.
- It forms part of anti-spam checks alongside SPF, DKIM, and DMARC.
- Ensures sender IP and domain alignment builds sender reputation and lowers email rejection rates.
- Network Diagnostics and Troubleshooting
- System administrators use rDNS to translate IPs to hostnames in logs, making error tracking and network monitoring easier.
- Many logging and monitoring tools rely on rDNS to improve the readability of reports by showing domain names instead of raw IPs.
- Security and Anti-Abuse
- Helps identify and block spoofed or malicious IP addresses attempting to masquerade as legitimate senders.
- Used in blacklists and reputation services to associate IP addresses with valid domain identities.
- General Internet Services
- Some protocols and services outside email validate connections using rDNS to improve trust or apply access controls.
Summary
- While rDNS is essential for email deliverability to reduce spam and improve acceptance, its use extends to network management, security, and diagnostics.
- Setting up a proper PTR record for your mail server IP is critical for email success but also benefits overall network transparency and trustworthiness.
Blacklisting
Both the mail server IP address and the domain name can get blacklisted, but typically:
- The mail server IP is the primary entity that gets blacklisted on public IP blacklists (DNSBLs). This happens because email servers check the sending IP reputation to block spammy or malicious sources.
- The domain itself can also get blacklisted, especially on domain-based reputation services or through DMARC and anti-phishing/spoofing protections when the domain is associated with spam or abuse. Some blacklists track suspicious domains as well.
Key points:
- If your mail server IP is blacklisted, emails sent from that IP are more likely to be blocked or flagged as spam regardless of the domain.
- If your domain is blacklisted, recipient servers may reject emails even if the IP is clean.
- Because many domains share mail sending IPs (like in shared hosting), IP blacklists are more common and impactful for deliverability.
- Domain blacklists tend to be used in anti-phishing, brand safety, and some spam filtering tactics.
Therefore, maintaining a good reputation for both your mail server IP addresses and your sending domains is essential for optimum email deliverability.
When you see a feature like “send from the domain IP only,” it usually means configuring your email sending so that the outbound emails originate from a specific dedicated IP address assigned to your domain or mail server rather than using shared or multiple IPs.
What this feature really does:
- It binds your sending to a fixed IP address tightly associated with your domain or subdomain used for mail.
- This IP is published in DNS, PTR (reverse DNS), SPF, and related email authentication records, ensuring proper alignment.
- This helps build a consistent sending reputation for that IP, separate from others.
- Sends messages that appear to come from your domain IP instead of generic shared hosting IPs or third-party providers.
- Typically used for better email deliverability, reputation control, and security.
Why this matters:
- If you use shared IPs, your deliverability can suffer if others on the IP behave badly.
- Using a dedicated domain IP as the sending IP centralizes reputation and makes troubleshooting easier.
- Ensures better DMARC SPF and PTR alignment, essential for avoiding spam blocks.
How it’s configured:
- You assign or own an IP.
- Set up DNS records (A, PTR, SPF) related to that IP/domain.
- Configure your mail server or sending platform to send only from that IP.
Note:
- The “domain IP” here does not mean the web hosting IP but the specific IP configured as your mail sending IP.
- Some platforms label this as “Dedicated IP” or “Dedicated sending IP.”
So in summary, the “send from the domain IP only” feature means sending emails from a dedicated fixed IP address affiliated with your domain, which improves deliverability by ensuring proper DNS and reputation alignment.
For best deliverability when setting your sending to use a dedicated domain IP (i.e., “send from the domain IP only”), here are the recommended best practices:
- Use a Dedicated IP Address
Ensure the IP is dedicated exclusively to your sending domain to fully control its reputation. - Proper DNS Setup
- Publish SPF, DKIM, and DMARC records aligned with the sending domain/IP.
- Set a matching reverse DNS (PTR) for the IP pointing to your domain or mail subdomain.
- Gradual IP Warm-up
When starting with a new dedicated IP, gradually increase volume over 4-8 weeks. Start sending small batches of emails to engaged recipients and slowly ramp up volume to build trust with ISPs. - Consistent Sending Volume
Maintain a regular and steady sending schedule. Avoid big sudden volume spikes or large send-lulls, which negatively impact reputation. - Segment Your Traffic
Use separate IPs for different traffic types (e.g., transactional vs marketing) to protect reputation and diagnose issues easily. - Monitor Reputation and Feedback
Regularly check blacklists, monitor DMARC reports, and take action on complaints or deliverability drops. - Clean Mailing Lists and Good Engagement
Target valid, opted-in recipients. Remove bounces and inactive users to maintain engagement metrics that favor inbox placement. - Work with Your ESP and ISP
Coordinate with your email service provider for setup help, and follow their warm-up and sending guidelines.
Summary
- Assign a dedicated IP for your domain mail sending.
- Set all relevant DNS auth records (SPF, DKIM, DMARC) correctly.
- Warm up the IP slowly and send consistent volumes.
- Use separate IPs for different email types if possible.
- Monitor reputation and respond quickly to issues.
For best deliverability, whether to select “send from domain IP” (dedicated IP) or pick another depends primarily on your sending volume and control needs:
Use Dedicated Domain IP (Send from Domain IP) if:
- You send a high volume of emails consistently (generally > 50,000 to 100,000+ emails/month).
- You want full control over your sending reputation without influence from other senders.
- You are prepared to warm up the IP gradually to build a positive reputation.
- You need precise troubleshooting and reputation management on IP-level.
- Your mail traffic is mostly transactional or critical emails needing strong deliverability guarantees.
Use Shared or Alternate IP if:
- You send low to medium volume emails.
- You want to avoid the responsibility of warming and maintaining an IP reputation.
- You prefer immediate sending capacity without warm-up.
- You benefit from the shared reputation of a vetted high-quality sender pool.
- You want to save on costs or complexities of dedicated IP management.
Summary of Pros and Cons
| Method | Pros | Cons |
|---|---|---|
| Dedicated domain IP | Full IP control, isolate reputation, best for high volume | Requires warm-up, consistent sending, ongoing maintenance |
| Shared or other IPs | Easier setup, lower cost, instant sending readiness | Reputation affected by others, less control |
In the case of using your own mail server, domain IP available:
- Select sending from your domain’s dedicated IP (mail server IP) if your volume is high enough and you want the best long-term deliverability.
- This ensures coherence in DNS records (SPF, PTR, DKIM, DMARC) and reputation control.
- If volume is low or setup complexity is a concern, a shared or alternate IP might be preferable.
Final advice:
Setup includes proper DNS alignment, PTR for your IP, and a staged warmup plan if choosing dedicated IP sending.
Mailster Multi-SMTP – How it distributed emails across the sending SMTP servers
When using Mailster with multiple SMTP servers, Mailster typically distributes the email sending load across the configured SMTPs in batches rather than strictly one email per SMTP server at a time. Here’s how it generally works:
- Mailster divides the total sending queue into batches and sends each batch through one of the configured SMTP servers.
- The amount of emails sent per SMTP server depends on each server’s configured limits and capacity, such as concurrent connections, sending rate, or throttle settings.
- Using multiple SMTPs helps spread out sending volume, avoids hitting per-server sending limits, and improves deliverability by distributing sending reputation.
- Mailster may rotate between SMTP servers per batch or per connection to balance load and minimize the risk of IP blocks.
- Exact batch size and distribution behavior can depend on Mailster’s version, multi SMTP add-ons, and specific configuration settings in Mailster’s delivery options.
Summary:
- Mailster does not send just one email per SMTP and switch; it sends batches through each SMTP up to their limits.
- It balances the queue by dividing it efficiently across the 4 SMTPs, respecting their individual limits.
- This optimizes throughput while managing deliverability and server load.
For precise control, check your Mailster multi SMTP plugin settings or documentation which may allow adjustment of batch sizes or concurrency per SMTP connection.