When sending email replies from the Inbox, there are times when emails fail to be delivered because of a problem with the recipient’s email account or provider. In these circumstances, we’ll surface the error message that their email provider sent back to us when we tried to deliver the email.

When the customer’s email provider sends back the error, we add a note to the conversation to surface the error message, along with the email address that couldn't receive your reply.

These are errors that Intercom has no control over. They represent a problem with the recipient’s email account or email provider, rather than a problem with Intercom. For example, if the recipient’s mailbox is full, or they have changed their email address.

When receiving errors like this, you can either try sending again later, or reach out to the customer through another address or channel if available.

Message delivery error notes wont re-open conversations if they are closed.

Common delivery errors

These are some error codes that are commonly received when an attempt to deliver an email fails, along with a description of what they usually mean.

Error code	What it means
421-4.7.28	This is a temporary rate-limit deferral from Gmail when Gmail detects an “unusual rate of mail originating from your SPF domain.” Gmail may defer delivery of your message with this code if its spam prevention systems see an atypical sending pattern — for example, a sudden increase in send volume from a new or low-volume domain. This is controlled by Gmail’s policies and not by Intercom. You can try sending again later, and ensuring your sending domain is authenticated (SPF/DKIM) and warmed up can reduce these rate-limits over time.
550	This can mean a variety of different things, such as the recipient address not existing, the email being filtered as spam, or the mailbox being full. The error message usually provides more detailed information about what’s going wrong.
550-5.1.1	This typically means that the email address you are trying to send to is not valid. The recipient may have closed their email account, or there may be a typo in their email address.
550-5.7.1	This error is often associated with issues related to email security, spam, or policy violations. The email may have triggered spam or virus protection filters, failed authentication checks, or been rejected due to policy violations related to the source or content. Ensure your sending domain is authenticated (SPF/DKIM) and ask end-user to check which rule could be rejecting these emails.
552-5.2.2	This indicates that the recipient's mailbox is full and cannot accept any more messages until space is freed up.
554-5.4.4	This typically means that the recipient's mail server could not find a route to deliver your email. DNS may be misconfigured on their mail server, there may be policy restrictions that block certain types of email, or there may be a temporary issue with the server or network.
554-5.4.7	This typically means that the recipient's mail server could not find a route to deliver your email. DNS may be misconfigured on their mail server, there may be policy restrictions that block certain types of email, or there may be a temporary issue with the server or network.

Email bounce list

After repeated permanent delivery failures, email addresses may be placed on a bounce list to protect Intercom's sending reputation.

What is the bounce list?

When an email address experiences a hard bounce (permanent failure), Intercom automatically adds it to a bounce list. This prevents future emails from being sent to that address, as continued attempts to deliver to invalid addresses can harm sending reputation.

Removing addresses from the bounce list

You cannot remove email addresses from the bounce list yourself. However, if you believe an address is incorrectly bouncing or if delivery issues persist after the underlying problem has been resolved, please contact our support team. We'll investigate and can remove the address from the bounce list if appropriate.

Important: Before requesting removal, make sure the underlying issue has been fixed (e.g., the email address is now valid, the mailbox is no longer full, or any server issues have been resolved).

Message failed to send error

If you are seeing a “This message failed to send. Click to retry.” error when sending an email or chat reply, this may be due to the user/lead being archived.