When a mail server gives Connection Refused, emails will come through in future attempts assuming the mail server comes back online. How well supported is this? Do all mail servers support this? Is it only the more robust ones? Is it defacto standard or spec?
As mentioned it is described at RFC 5321 but unfortunately the implementation in SMTP Servers differ. There are at least these scenarios:
Some servers even distinguish between 4xy retry codes, connection lost, timeouts, network unreachable and other temporary errors and behave differently on each of the condition.
To sum it up: Even if there is a standard you have to admit that can't relay on the implementation.
SMTP servers usually try to deliver message for days. It is specified in RFC 5321 Simple Mail Transfer Protocol, Section 6.1 Reliable Delivery and Replies by Email and Section 4.5.4 Retry Strategies.
Added excerpts from RFC: