Delay in sending mail postfix queue
I have a mail server that periodically connects to the Internet via modem dialup and before was able to send emails with attachments of 300kb and 500kb in just only 1 or 2 minutes, I have not made any change in postifx and now I take longer than normal to out postifix queue, sometimes it took more than 10 min to deliver an email of just 300kb, if this is in queue with others mails containing only text accompanying the rest out normally while the of 300kb containing an attachment is delayed more than usual. previously this was not happening and all the emails was sending very quickly.
any suggestion of what is going on here? this can be caused by a very long postfix maillog file that needs to be rotated, or reconfig the mastercf ? please help