I suspect in looking for alternatives for plus addressing, you are mixing this up with a special feature Fastmail provides for POP retrieval (being able to retrieve messages from different folders than just the regular Inbox) in combination with the ability to use alternative usernames when accessing Fastmail services (such as IMAP) from external mail clients. It allows, for instance, a POP username setup of userid+SomeFolder#fastmail.com to extract messages from the SomeFolder folder of
userid@fastmail.com.
This is all documented in
https://www.fastmail.com/help/techni...sandports.html
As far as I know, there is no way to use plus addressing without the "+" except subdomain addressing which is usually the superior method. I am quite surprised there are email services that reject the use of destination addresses of the form
whatever@username.fastmail.com.