Using Exchange 2003 SP2 on an SBS2003 SP2 server. Mail is sent/received via SMTP. Receiving of emails is 100% fine, but sending of emails seems to have developed a problem that I can see no reason for. A good proportion of email gets sent okay, but also quite a few become delayed and sometimes do not get delivered at all. I have tried configuring the SMTP connector to send via DNS and via our ISPs mail relay, but I have the same problem, particularly with emails that have attachments. My domain does not appear to be blacklisted, or on any spam lists. I also have GFI Mail Security running (mail virus checking software) and GFI Mail Essentials (spam checking software) but these are only checking INCOMING mail. There have been no major changes to the system or network, and I'm starting to pull my hair out in frustration at not being able to find a reason. Any help or advice would be much appreciated.