Mail seems to be sending mail but in fact it is not

Hi everybody!

When sending mails using Mail.app everything seems just fine and basically it works well, but sometimes the emails don't reach the recipients.
No popups, no messages, nothing in Console.app, no errors at all. Messages go but don't get to recipient(s).

I have Leopard 10.5.6, all updates are installed. Mail's version is 3.5.

Our setup has ~25 SMTP accounts (so, no IMAP), the problem seems to spread across them all (once it happens at all).

The emails are usually being sent to 2 persons: one address goes to the To field, another one - to CC.

If I restart the Mail.app the problem goes away but it will happen next hour.. or tomorrow..
Once I remember the email didn't reach the recipients right after the computer boot up and we've started the Mail.app..
Internet connection is alright; we also have an internal SMTP server within the office, same problem with it.

If I know the mail didn't really go I just restart the program (i.e. Mail), go to Sent folder, pick the message from there, click Forward or Redirect and send it again, and it goes this time. That means that everything with settings, addresses etc. is fine.

I've run Disk Utility - Verify Disk (no problem) and Verify Disk Permissions ("group differs on private/etc/cups, should be 0; group is 26")..

When googling this problem I saw somebody mentioned ~/Library/Caches/Mail/ folder; I don't have such at all.

I've also tried to boot the computer holding Shift to get to Safe boot, then restarted to normal. Of course, it could help that time but we still have this problem at the moment.

Oh, the typical email is an empty email with 1-10 PDF files attached. All names are in English. In the Edit - Attachments menu I have only one option checked: Always send Windows-Friendly Attachments.

Any ideas will be highly appreciated.
Thanks for your input.

 

Dear MisterMe,

Thank you for your reply.
No, the problem is local. We have about 20 computers in the office, all of them work with our mail servers just fine.
Moreover, we have mail servers in the internet running qmail as well as local one (i.e. which is connected to LAN) running Kerio MailServer on Windows 2003 and we have them for many years so far.

Even when we previously used Thunderbird on this particular computer we never had such a problem with sending mail, this is only Mail.app problem. I agree that it might be caused with a third-party application/issue.

Please let me know what kind of an additional information you need to further investigate the issue (dumps/logs etc)

Thank you.