Are Sendgrid MTA retries damaging my sender reputation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 26 Apr 2025
Updated 18 Aug 2025
7 min read
When you send emails through a platform like SendGrid, you'll inevitably encounter messages that don't get delivered on the first try. These are often categorized as deferrals, which the Mail Transfer Agent (MTA) will then retry. It's a common part of the email sending process, but it raises a critical question: Do these ongoing retries, sometimes lasting for days, negatively impact your sender reputation?
The short answer is, it depends. MTA retries are generally designed to overcome temporary issues like a recipient's inbox being full or an ISP (Internet Service Provider) throttling incoming mail. In most cases, a normal pattern of retries by a reputable ESP, such as SendGrid, isn't inherently damaging to your sender reputation. In fact, it can sometimes be a positive signal that you are making a good faith effort to deliver the message.
However, an unusually high volume of persistent deferrals can certainly be a red flag. This might indicate underlying issues with your sending practices or your email list hygiene. Understanding the nuances of these retries is key to maintaining a healthy sender reputation and ensuring your emails reach their intended inboxes.
Understanding MTA retries
When you send an email, it goes through a series of servers, known as Mail Transfer Agents (MTAs), before reaching the recipient's inbox. Sometimes, a receiving server might temporarily reject a message, issuing a deferral notice instead of a hard or soft bounce. This signals a transient issue, and the sending MTA will queue the message for later retry attempts. These retries are a standard part of email protocols, designed to ensure delivery resilience.
Common reasons for deferrals include recipient mail server being temporarily unavailable, greylisting policies, rate limiting by ISPs, or a full recipient mailbox. For instance, ISPs like Yahoo or AOL might implement throttling if they detect a sudden surge in mail volume from a particular IP address or domain. The MTA will then patiently retry over a period, which could range from hours to several days, until the message is either successfully delivered or a permanent bounce code is received.
It’s important to distinguish between a deferred email and a soft bounce. While both represent temporary delivery failures, a soft bounce usually results in the email being returned to the sender with a specific error code after a certain number of retry attempts have failed. A deferral, on the other hand, means the email is still in the process of being retried by the sending MTA. You can learn more about how soft bounces affect deliverability and sender reputation.
Typical MTA retry behavior
Gradual retries: MTAs typically increase the time between retry attempts for a given message.
Time limits: Most ESPs have a maximum retry duration (e.g., 72 hours) before marking an email as a soft bounce.
Normal traffic: A certain percentage of deferrals is normal and doesn't immediately signal an issue.
How retries affect sender reputation
The key concern for senders is whether these retries negatively affect their sender reputation. Generally, isolated deferrals and their associated retries do not directly harm your reputation, especially when using a shared IP provided by an ESP. ISPs understand that temporary issues occur, and a sending MTA that retries messages actually looks more legitimate than one that gives up immediately, which can be a characteristic of malicious senders.
However, a sustained high rate of deferrals, particularly from major ISPs like Yahoo and AOL, can be a symptom of a deeper problem. If you're consistently seeing a large number of deferred messages, it might indicate that the receiving server is explicitly throttling your mail due to suspicious activity, poor engagement metrics, or a low domain reputation. In such cases, the repeated connection attempts, while not inherently negative, won't help improve a tarnished reputation and could prolong recovery efforts.
Your sender reputation is a complex score determined by various factors, including bounce rates (both soft and hard), spam complaints, engagement, and whether your IPs or domains appear on any blocklists (or blacklists). If deferrals lead to an elevated soft bounce rate, or if they are a symptom of ISPs pushing back against your volume, then yes, they are indirectly damaging your reputation. It's crucial to understand how IP reputation versus domain reputation affects your overall email performance.
Normal retry patterns
Occasional deferrals: Seen across various ISPs and not concentrated at one.
Resolution within hours: Most deferred messages are delivered within a short period.
Diverse causes: Varied deferral codes, indicating different temporary issues.
Reputation impact
No negative impact, as it reflects standard email flow and resilience. A good ESP manages these for you.
Problematic retry patterns
High volume at one ISP: Many retries specifically from Yahoo or AOL, for example.
Extended deferrals: Emails remaining in deferred status for days without resolving.
Consistent causes: Repeated deferral codes pointing to persistent issues, like throttling.
Reputation impact
Indicates underlying deliverability issues such as excessive volume, poor list quality, or low engagement, which can damage reputation.
Managing SendGrid retries and sending volume
Since SendGrid manages its own MTAs, you generally don't have direct control over their retry policies. However, you can influence the factors that lead to deferrals. If you're seeing high deferral rates, especially during a migration or a sudden increase in send volume, it often points to issues with your sending practices rather than SendGrid's retry logic itself.
One of the most common causes of deferrals when warming up new IPs or domains (like when moving from Amazon SES to SendGrid) is sending too much volume too quickly. ISPs interpret sudden spikes in volume as suspicious, leading to throttling and deferrals. You can find out more about recommended email send volume increases to mitigate this risk.
To prevent excessive deferrals, focus on gradual volume increases and maintaining a highly engaged email list. Segment your list to target your most engaged recipients first, especially during a warming period. This sends positive signals to ISPs and helps build your domain's reputation effectively.
Controlling volume and list hygiene
Gradual ramp-up: Increase sending volume slowly, especially when migrating or using new IPs. This helps build trust with ISPs.
Segment by engagement: Prioritize sending to your most engaged subscribers first. These recipients are more likely to open and click, providing positive feedback to ISPs.
Clean your lists: Regularly remove inactive or unengaged subscribers. This reduces hard bounces and spam trap hits.
Best practices for SendGrid deliverability
While you can’t directly control SendGrid’s MTA retry logic, you can certainly monitor your deferral rates and other key metrics to identify if there’s a deeper issue at play. Pay close attention to your SendGrid activity logs for deferral patterns, especially if they are consistently high from a specific domain or ISP.
Look at the ratio of deferred emails to delivered emails. A disproportionately high deferral rate could indicate that your email program is being throttled or that the recipient servers are highly suspicious of your sending patterns. High deferrals can sometimes precede a blocklist (or blacklist) listing if not addressed promptly. Regularly checking blocklist checker tools is a good proactive measure.
Ultimately, improving your engagement rates is one of the most effective ways to mitigate deliverability issues, including deferrals. ISPs prioritize mail from senders whose recipients consistently open, click, and reply to their emails. A strong engagement history signals to ISPs that your mail is wanted, leading to fewer deferrals and better inbox placement. You can explore more about how email sending practices impact domain reputation.
Deferral reason
Potential impact
Actionable insights
ISP throttling
High volume for reputation or new sender
Slowly increase volume, prioritize engaged users
Temporary server issues
Minimal, usually resolves on its own
Monitor logs for resolution, no immediate action needed
Recipient mailbox full
Can indicate an old or inactive list
Implement list hygiene, re-engagement campaigns
Greylisting
Standard anti-spam measure, usually harmless for legitimate senders
Ensure your ESP handles retries correctly, which SendGrid does
Views from the trenches
Best practices
Continuously monitor your deliverability metrics, especially bounce rates and deferrals, to catch issues early.
Segment your audience and prioritize sending to highly engaged subscribers, particularly during IP warming periods.
Maintain a clean email list by regularly removing inactive or invalid addresses to reduce bounce rates.
Common pitfalls
Ramping up email volume too quickly when migrating to a new ESP or IP, leading to ISP throttling.
Not regularly cleaning email lists, which can result in high soft bounces and damage to sender reputation.
Ignoring consistent deferrals from specific ISPs, which can indicate a larger deliverability problem.
Expert tips
SendGrid's internal reputation metrics are not external, so always check third-party blocklist services.
When faced with deferrals, it's generally not about SendGrid's retry policy but your sending behavior.
If moving traffic, use past engagement data to target engaged segments during the warming phase.
Expert view
Expert from Email Geeks says that a normal amount of retries is expected, and an MTA not retrying deferred mail can make the sending server appear suspicious, as hackers tend not to retry deferrals.
2025-01-09 - Email Geeks
Expert view
Expert from Email Geeks says that while direct MTA access allows reducing connection rates to Yahoo/AOL, with SendGrid, it's necessary to reduce total aggregate volume or throttle the rate at which emails are passed to SendGrid for delivery.
2025-01-09 - Email Geeks
The bottom line
SendGrid MTA retries are a fundamental part of reliable email delivery, designed to overcome temporary roadblocks. While normal retry behavior doesn't inherently damage your sender reputation, persistent and high volumes of deferrals are a strong indicator of underlying issues with your email sending program. Focusing on controlled volume increases, maintaining a clean and engaged email list, and continually monitoring your deliverability metrics are the most effective ways to ensure your emails reach the inbox and protect your sender reputation.
SendGrid, isn't inherently damaging to your sender reputation. In fact, it can sometimes be a positive signal that you are making a good faith effort to deliver the message.
Amazon SES to SendGrid) is sending too much volume too quickly. ISPs interpret sudden spikes in volume as suspicious, leading to throttling and deferrals. You can find out more about recommended email send volume increases to mitigate this risk.
