Outlook often stops sending Mail

Hi!

After a while Outlook stops sending Mails and they get stuck in the Outbox folder.

After a restart we are usually able to send a few mails but then one gets stuck again and outlook stops sending.

This happens on two different installations, one connected to a "regular" Mailserver via Pop3/SMTP, the other to an Exchange Server.

We tried everything suggested in

http://support.microsoft.com/kb/2434474/en-us

and

http://support.microsoft.com/kb/2360509/en-us

but none of  it helped.

Even after a complete reinstall of Office 2011 as described in

http://support.microsoft.com/kb/2398768/en-us

the problem persists.

Any suggestions how to fix it or how to find out why this is happening is highly appreciated.

Thanks in advance.