Dealing with a business here which has had an ongoing problem with their email for over 2 years now.
Emails configured in Outlook 2007 using IMAP.
This is a branch office with approx 8 seats. AFAIK, there is no MS Exchange involved.
Problems sending out.
In a nutshell, you could write 5 Test Messages - press and receive and none of them will send. They will be stuck in Outbox. Then half an hour later you can press on "send/receive" and they all go out.
This problem is not just affecting one use but 3 other users as well.
The max upload load speed is .45Mpbs.