A client asked me earlier this week what SLAs ISPs provided for email delivery. The short answer is that there isn’t a SLA and that the only guarantee is that the email will get there when it gets there.
But as I was mentioning this to Steve, he pointed out that there was a recent change in the RFCs for email. In both RFC 821/2 and RFC 2821/2 (the original email related RFCs and the update in the early 2000’s) the RFCs stated that once a receiving MTA accepted an email that that MTA was required to either delivery the mail or generate an asynchronous bounce. While this isn’t a standard SLA, it does mean that a 2xy response after DATA meant the email would either be delivered to the user or be sent back to the sender. Despite the RFC requirements some receivers would still drop mail on the floor for various reasons, sometimes intentionally and sometimes not.
RFC 5321/2, the current SMTP standard, still says that once a server accepts the mail it must not lose that mail ‘for frivolous reasons.’ The RFC goes on to admit, though, that in recent years, SMTP servers are under a range of attacks and dropping mail on the floor is not frivolous in those cases.
 

Delivery Improvement • RFCs • SMTP