Tips for troubleshooting mailbox provider deliverability problems

It can sometimes be difficult to know where to start when you see deliverability problems. Whether the deliverability problems are widespread or focused on only a few mailbox providers, these tips can help you determine a cause. 

Initial troubleshooting steps

  1. Determine the scope of the deliverability problems  

Are the problems:

    • For one campaign or for all campaigns?
    • For a specific IP address or domain or all IP addresses or domains?
    • Only for your promotional campaigns or do they only affect transactional or other triggered messages?
    • For a specific email stream such as a newsletter or product line?
    • Occurring on a specific day or time of day?
    • For just one mailbox provider, or do you see similar problems at other mailbox providers?
    • For a specific segment of subscribers or all subscribers?
    • For a specific brand you own or all brands?
    • For a specific email template or content? 
  1.   Determine if your IP addresses are on Blacklists 

If your IP address or domain is listed on a blacklist, refer to the Blacklist’s listing criteria and fix the cause. Return Path Platform alerts you to blacklist issues but you can also check your SMTP logs for error codes and descriptions indicating a blacklisting. Not all Blacklists are created equally as many of them are minor and have little to no effect to your deliverability. Important blacklists such as Spamhaus, Spamcop, URIBL and SURBL can have the biggest negative impact to your inbox placement if your IP or domain is listed. Typical reasons for being blacklisted are because of a high number of complaints and sending to spam traps. 

  1.   Check your SMTP logs for clues regarding the issue

If your email is being throttled or blocked, check your SMTP logs for a response code indicating a possible cause. Many mailbox providers provide a response code, a short description and a link to their postmaster page you can use to troubleshoot. If you use an Email Service Provider (ESP), ask them if they have a report you can run that lists these response codes. 

  1.    Check your sending processes 
    • Ensure the complaint handling process is working and complainers are added to a suppression list. Be sure you are signed up with all available complaint feedback loops.
    • Ensure the bounce processing process is working and unknown users are suppressed or removed after one bounce.
    • Immediately honor all unsubscribe requests to ensure future email messages are not marked as spam.

Additional troubleshooting tips 

  • Ensure your IP is configured with a valid reverse DNS record: Your IP address needs a valid, non-generic reverse DNS (PTR) record pointing to a domain.  A missing or generic ( reverse DNS record can cause deliverability issues.
  • Determine if you in compliance with sending best practices: If you are having issues at one or more mailbox providers, make sure you are in compliance with their suggested sending best practices. If you need to use their public escalation process, they may be more likely to be helpful if you are following their best practices. Otherwise you may just get a generic response from their support team that points you to their best practices web page.
  • Check Microsoft’s Smart Network Data Services (SNDS) data: Microsoft’s SNDS data is free and provides data on complaints and spam traps for Microsoft domains. Even if you are not encountering delivery issues at Microsoft, SNDS complaint and spam trap data may help point you in the right direction.
  • Look for spikes in your data: Large, sudden increases in complaints, unknown users, spam traps and unsubscribe requests along with declining response metrics can help you pinpoint potential causes for deliverability problems.
  • Determine if you increased the frequency of email sent to subscribers: If you started sending more frequently for the latest holiday or just in general, it could be leading to more complaints, unsubscribes, lower engagement rates and more messages that are deleted without being read. You may need to send less frequently in order to optimize the frequency of email sent to your subscribers.
  • Check your Certification data: If you participate in Return Path’s Certification Whitelist program, the data can be useful in identifying potential causes of your deliverability problems.
  • Review your list acquisition and list hygiene processes: Spam traps are an important reputation measure for many mailbox providers. Most mailbox providers will bounce back inactive addresses as unknown users for 3-9 months before a recycled spam trap is created.
    • Find out if you recently acquired email addresses from a higher risk source such as a list purchase or e-append and stop sending to those email addresses. If you have a co-registration partner, you may want to temporarily stop sending to addresses acquired from them until you can verify that the email addresses opted in to your email program.
    • Look for large spikes in the number of new subscribers over the past 90 days. If you average 100 new subscribers a day and you see that number suddenly spike up to 1000 over a few days, it may indicate list poisoning. Suppress those email addresses until you can verify they are real subscribers.
    • Ensure your list hygiene plan is working as expected. If you don’t have a plan, you should create one that makes sense for your business. Sending to a large number of subscribers that don't engage with your email can cause deliverability problems.
  • Determine if your content contains a spammy Cloudmark fingerprint: Many mailbox providers use Cloudmark in their filtering decisions. After identifying and fixing the cause for the spammy fingerprint, submit a reset request. If you don’t fix the cause of the issue, your content is likely to be marked with a spammy fingerprint in the future.
  • Determine if you sent a large volume of email in a short period of time: Large volume spikes may be perceived as spam. If it was a one-time event, monitor performance over the next few days to see if deliverability normalizes. Spread out future large volume sends over time to avoid being perceived as a spammer.
  • Determine if you sent to a suppression list: If you did send to a suppression list, your performance should normalize as your metrics return to normal levels. Many senders send to a suppression list by accident, but some senders send to a suppression list to try and re-engage old subscribers, which usually results in deliverability problems.
  • Determine if  you are sending from a new IP address or domain: Many deliverability problems occur when a new IP address or domain is not warmed up properly. If you did warm up the IP address and domain properly or are in the process of warming them up, you may still encounter deliverability issues while your sending reputation is built. It is important to note that sending success from a new IP or domain is closely tied to the quality of data you have. If you have highly engaged subscribers, you are more likely to be perceived as a legitimate sender. If your subscribers do not engage with you often, you are more likely to be perceived as a spammer.
  • Determine if your email authentication is working properly: A misconfigured SPF, DKIM or DMARC record can result in email being filtered or blocked. Check for the “Authentication-results” in your email headers using Inbox Monitor to determine if there are any verification failures. Return Path’s Smart Alerts can also help you identify verification failures and give instructions to fix the issues. A DMARC record with a p=quarantine or p=reject policy flag can cause spam folder placement or rejected messages from your domain if SPF and DKIM are failing verification.
  • Check your connection and throughput settings: Mailbox providers will throttle your email if you are sending too much email to their servers. Reduce the connection and throughput settings if throttling occurs.
  • Check your retry settings: Increase the time between retries if experiencing throttling issues. You may want to limit the number of retries per message to 2 or 3 temporarily if needed until you can isolate a cause of the deliverability problems.
  • Check your content: Although most deliverability problems are due to sending reputation factors, many mailbox providers still factor content in to their filtering algorithms.
    • Run you content through Inbox Preview and make sure it is rendering properly across all of the email clients. Improperly rendered campaigns may lead to lower response rates and higher complaints.
    • If you send one big image with no text in the message body, then try increasing the amount of text in the message body to have greater balance of text to image.
    • Ensure all URLs link to a valid web page, are not broken and don't send someone to a website hosting malware.
    • Avoid using link shorteners like as these domains consistently show up on blacklists.
    • If you copied code from Microsoft Word or Publisher, you may have extra HTML code that is triggering a flag at the mailbox provider.


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