Outlook 2007 - Mail cannot be sent, when a public folder is attached to outlook

Hey Folks,

I have a weird issue that troubles some of my co-workers.

We're using an Exchange 2013 als Mail Server and Outlook 2007 as Client. Some users, i have no i clues why only some are affected, running into the issue that an email cannot be sent and got stuck in outbox, as long as a public folder is attached within outlook 2007. If the user disconnects from the public folder, outlook sends any email immediately. I was able to recognize a huge amount of connection tries within the outlook connection overview, but the try is never successful, so the counter rises.... The version of Outlook is MS Outlook 2007 (12.0.6680.5000) SP3 MSO (12.0.6683.5000)

Does anyone got this issue too and has a solution?

Thanks,
Sascha