Comment by bbarnett

SHOULD is not MUST. These capitalized terms are have very specific meanings in RFCs, see RFC https://datatracker.ietf.org/doc/html/rfc2119. SHOULD is:

3. SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.

MUST is a requirement. You left out the "however" part:

In general, the retry interval SHOULD be at least 30 minutes; however, more sophisticated and variable strategies will be beneficial when the SMTP client can determine the reason for non-delivery.

There's absolutely nothing wrong with a fine tuned backoff. I am not saying the specific backoff discussed by GP is best, merely that 30 minutes is absolutely not a requirement, and in fact, discussed in tandem with the fact that "more sophisticated strategies" are actually beneficial.

The RFC does not agree with you. Partially quoted as you have, does not help.