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I am a bit rookie with sieve. Some time ago I created a sieve pipe that would sometimes alter alter the subject and move a message to a different mailbox. Currently this results in that a date changes of this message while I don't see anything in the source altering a date.

What could be causing this and how to work around this?

can you show info that might be useful to understand the issue you are facing? For example, what are the contents of the sieve script? When is the script being executed (for example on mail delivery by lmtp/lda)? When you say the date changes of the message can you give an example of what you are seeing?

I have been checking this a bit furhter and it is seems this behaviour is limited to the apple mail client ... I can't replicate this behaviour at least via the webmail interface.