This is not a matter of how to set the priority of an email, but rather a matter of why the values are what they are, and where those are specified.
Pretty much every SMTP Library that I've seen states that there's 5 priorities:
However, none of them actually cite their sources, which leaves me to wonder if this is just something globally understood, or if there's an actual specification.
Could someone please explain to me when these priorities where first enforced, and properly specified? I tried looking through a bunch of RFC stuff, but I all I found was how to handle message priorities, not what the values were.
Personally, I'm implementing the SwiftMailer PHP library. You can find it's priority settings here. However, as stated, there is no reference to back its claim.
https://www.rfc-editor.org/rfc/rfc6710.html#page-4
... when relaying the message to the next hop or delivering ...
... an SMTP client that has more than one email to send at a given time sends those with a higher priority before those with a lower one. Additionally, the retry interval and/or default timeout before a non-delivery report is generated MAY be lower (more aggressive) for messages of higher priority. ...
Delivery time constraints...
Priority of email was originally related to hosts that were not always connected. This caused an undeliverable after xx attempts reply to the originator to be sent after varying number of days depending on priority.