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can a filter append text to the subject line of the message?
Proposed
Pobox.com can do this, but it is getting merged into Fastmail, which cannot do this.
I like this idea 
No connection
Real-time notifications may not work
Mailbox.org seems to support editheader and variables extension. So I guess it should work if you edit the sieve script via manage-sieve. Good luck. Maybe you can report back.
https://superuser.com/questions/1502588/how-to-change-a-messages-subject-in-a-sieve-rule
Mailbox.org seems to support editheader and variables extension. So I guess it should work if you edit the sieve script via manage-sieve. Good luck. Maybe you can report back.
https://superuser.com/questions/1502588/how-to-change-a-messages-subject-in-a-sieve-rule
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