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What would be the best way to 'throttle' dovecot/imap processes of specific users?

Can you please elaborate a bit how you would like to ‘throttle’ the processes? Give it less CPU time?

That you can probably achieve by returning service/imap/executable="/usr/bin/nice -n 10 /usr/libexec/dovecot/imap" from users for the throttled users.

Thanks Sami, I was experimenting with some external script that applied cpulimit and ionice to processes. But this suggestion is of course much nicer.