[Dovecot] Size of Mailbox affecting the sending of mail?

[Tim Smith]

Hi,

I didn't know where to post this one so I'll start off with you guys and 
then try Postfix if I'm barking up the wrong tree!

Having set up my mail server (Dovecot/Postfix), users are experiencing 
long delays (a couple of minutes) when sending mail from mail client 
such as Thunderbird - this increases with attachments. Having had a 
brief discussion with someone, they mentioned that the reason that this 
may be to do with the size of the mailbox. I couldn't see the rationale 
behind this unless Dovecot is syncing the mailbox after every sent mail 
(due to possibly saving the sent item?) The mail is being delivered 
successfully but the amount of time it is taking to complete the action 
is far too long!

Sorry this is a bit vague but any advice on trying to diagnose the 
problem would be appreciated.

Thanks in advance!

Tim