Why is a Sender Reputation Score (SRS) important?
As an example, you have a survey and are sending out 10,000 email invitations; out of those 10,000 emails, you have a large amount of email addresses that are invalid or or do not exist anymore, causing 15% of them to bounce. This causes a high Bounce Rate. Additionally, the body of the email invitation as well as the subject line (e.g., "Provide feedback and get a $10 gift card) are poorly formed. Because of this, you are receiving some complaints, which results in a high Complaint Rate of 3%. This means that recipients are marking your email invitation as unwanted/spam. After you send out the first batch of 10,000 email invitations, you send out 10,000 more email invitations the following day and 10,000 the day after that. You notice that you are continually getting complaints, more bounces, and less responses.
In Agile Research, you can check your own account Sender Reputation Score (SRS), which shows the health of your account. Sending out many emails with many bounces and complaints will affect your sender reputation, causing you to miss your audience. You will not be able to gather data or get feedback if you have a poor sender reputation.
Email providers such as Google and Outlook have sophisticated algorithms that analyze each email batch sent, and high bounce and complaint rates will trigger their filters. This leads to increased scrutiny and, in many cases, filtering out of emails from that sender. The following sections break down how providers respond to these metrics and what typically happens in future sends.
Bounce rate and its impact
A bounce rate of 15% (indicating that 1,500 out of 10,000 emails could not be delivered) is seen as problematic by most ISPs. When bounces occur at this level, email providers interpret it as a sign of poor list hygiene, assuming the sender might be careless about updating their contact lists or is perhaps using outdated or non-consented email lists. High bounce rates suggest the sender is not utilizing best practices for recipient engagement and list management.
For subsequent sends, this bounce rate will prompt ISPs to be more cautious. This impacts deliverability in the following ways:
Increased filtering to spam — ISPs will begin to see emails from this sender as higher risk, and they may redirect subsequent emails to the spam folder. Gmail, for instance, uses bounce rates as a key factor, and repeated bounces significantly increase the chances of an email being filtered out.
Potential throttling or blocking — With persistent high bounce rates, some ISPs, such as Outlook, may apply throttling, slowing the rate at which emails are accepted. In extreme cases, they may even block emails from the sender’s domain or IP if bounce rates don’t improve over time.
Complaint rate and its consequences
A complaint rate of 3% (where 300 out of 10,000 recipients mark the email as unwanted or spam) is also a strong negative signal. ISPs monitor complaint rates very closely, as complaints indicate that recipients find the emails intrusive, irrelevant, or potentially harmful. The industry benchmark for complaint rates is generally below 0.1%, so a 3% complaint rate is far above what is considered acceptable. This can result in the following:
Automatic routing to spam — Once the complaint rate crosses a certain threshold, ISPs are likely to automatically flag future emails from this sender to go directly to the spam folder. Google, Yahoo, and Outlook all rely heavily on user feedback metrics like complaints and adjust their filters accordingly.
Reputation downgrades — ISPs use these complaints to downgrade the sender's reputation. A low sender reputation drastically decreases the likelihood of future emails reaching the inbox, as algorithms treat the sender as a potential source of unwanted content.
Content and repeated patterns
When the subject line and body content remain the same across email batches, ISPs view this as repetitive. And if past sends with the same content generated high bounce and complaint rates, subsequent emails with the same content are flagged as likely spam.
Providers like Gmail and Outlook analyze patterns in subject lines and content, especially for senders with a history of poor performance metrics. This repeated use of problematic content, coupled with high bounce and complaint rates, triggers automatic spam filtering and increases the chance that future batches will not reach the inbox.
Impact on future sends
The following lists some ways that major providers are likely to handle the next batch of 10,000 emails from this client:
Higher likelihood of spam filtering — Providers like Google and Outlook may send these emails straight to the spam folder for a majority of recipients, especially those who have not opened previous emails from this sender. Even recipients who would normally receive emails in their inbox could see them in spam because of the sender's deteriorated reputation.
Decreased engagement metrics — As more emails end up in spam, engagement rates (open and click rates) drop, which in turn feeds back into ISPs’ algorithms, reinforcing the spam filtering.
Potential blocking or domain blacklisting — If high bounce and complaint rates persist, ISPs might add the sender’s domain or IP to their internal blacklists, blocking emails entirely. This can be particularly strict with providers like Yahoo and Outlook, who are known for their firm stance on high-bounce senders.
Steps to improve deliverability and avoid filtering
To prevent emails from being automatically filtered out or blocked, it is crucial to address the root issues causing the high bounce and complaint rates, which includes the following:
Clean up the email list — Remove invalid or outdated email addresses to reduce the bounce rate significantly. Using list hygiene tools or verifying emails before sending can help maintain a cleaner list. Use tools like Neverbounce.com or alternatives to it.
Reduce complaint triggers — Make sure the content and subject lines are relevant, personalized, and not too sales-focused or aggressive. Also, ensuring recipients have opted in to receive emails from this sender can reduce complaints.
Segment and engage — Send emails only to the most engaged segments or subscribers who have actively opened or clicked on emails recently. This will help boost engagement rates and lower the risk of complaints.
Test with smaller batches — Send smaller batches initially to gauge response and adjust accordingly. This approach helps limit the negative impact of a large batch if issues arise.
Update content and subject lines — Try modifying the content and subject line, especially if previous messages have been flagged. This prevents ISPs from pattern-matching based on prior complaints or negative engagement.
By actively addressing these factors, future batches will have a better chance of reaching the inbox. Major providers’ algorithms respond well to proactive measures and improvements in engagement and reputation over time.
See Sender Reputation Score (SRS), High bounce rate warning, and High complaint rate warning for additional information.