Incoming email messages to your mailbox can be rejected or delayed for a number of reasons, including mailbox full issues, sender reputation issues, email security and high traffic, and on rarer occasions systems maintenance or service issues.
Depending on the nature of the issue, our servers will usually send back a response to a sending server indicating what it should do. There are three main categories:
1) Message should not be accepted (5XX)
This typically includes Mailbox Full, Invalid recipient, Spamfilter rejections etc.
In these cases, our server will usually respond to the sending server with a 5XX status and error message and the sending server should not retry but send a bounce message back to the sender so that they understand the reason the message was not delivered.
2) Message should be retried (4XX)
This typically includes Spamfilter greylisting, system maintenance, temporary service issues, high traffic issues
In these cases, our server will usually respond to the sending server with a 4XX status and error message and the sending server should retry sending the message.
3) Cannot contact server
This can happen most commonly with DNS issues on customer domain, but could also be due to system maintenance or service issues.
In these cases, the sending server should retry sending the message.
Message retries from sending server
When a sending server cannot contact our server to relay a message, or is instructed by our server to retry (4XX status) it should retry sending the message with reducing frequency up to its retry limit which is recommended to be 4-5 days (RFC 5321). Initially, after a number of retries, the sending server may send a "delivery delayed" message back to the sender, but continue to retry sending. If the message is not delivered after its retry limit, it will cease sending and send a non-delivery report back to the sender so they know the message was not received.
Although this is the recommended action, it is not an absolute requirement, and senders may define their own sending frequencies and retry limits. Some sending systems may decide to cease retrying before the recommended 4-5 day limit and send a non-delivery report back to the sender earlier.
