I run an MTA sending billions of emails daily. Most mail is delivered in seconds, but there is a long tail that can take days (myriad of reasons involving recip

I run an MTA sending billions of emails daily. Most mail is delivered in seconds, but there is a long tail that can take days (myriad of reasons involving recipient mail servers, dns, sending behavior , etc). It would not be fun to have your log in attempt come in a couple days later. 99.9%+ you will be just fine, but how do you handle those people who fall through into long-delay land?