[MlMt] Deleting duplicate sent mails - rule

[John Man]

Hi,

When I send an e-mail message, I get two messages in my sent mailbox.

One is added by MailMate and the other by Exchange Online which I see 
(and this only started when - I believe - I enabled DKIM and presumably 
Exchange Online is adding the DKIM signed message).

 From:

https://lists.freron.com/mailmate/2016-April/006035.html

Benny wrote:

"As a workaround you can setup deletion of sent messages yourself. Add a 
rule to the “Sent Messages” mailbox which deletes any message which 
has a “Relayed” header (Relayed exists). This is a virtual header 
which only exists in MailMate and not in the FastMail copy."

I did this but the rule doesn't seem to automatically work on incoming 
messages.  It only works when I manually select the messages and 
manually apply the rule ?

Kind regards
John