Cannot click send button in Outlook (+ Exchange) for unknown addresses

Question

I have a very unusual problem.

I have Outlook 2010 connected to Exchange 2010. This can send emails perfectly to known addresses (that is, addresses in the address book or ones that have been sent to previously).

However, if I put in an address that is unknown, I cannot actually click the Send button in Outlook. (it simply does nothing). Corresponding to this I get errors in the Event Log for each Send click stating "The connection to Microsoft Exchange is unavailable. Outlook must be online or connected to complete this action.". However, Outlook shows as connected the whole time, pings do not break, and I have no reason to suspect it has lost connection.

To further complicate matters, Outlook is fine on all other PCs, and this was all perfect until I installed BitDefender on the PC in question and the Exchange Server. Outlook was still fine on these other PCs while BitDefender was installed, but I have removed it from the PC in question and the Server just in case (no success).

Summary: Outlook encounters Exchange connectivity issues when sending to unknown (new) email addresses that prevent the Send button actually working at all. This is isolated to one machine and occurred after installation of AV/Firewall software which has since been thoroughly removed.

If you have any potential solutions I'd love to hear them, as I will be resorting to reformatting the PC in question, and probably removing Exchange because I'm sick of its issues if I cannot resolve this soon.

Big thanks for any help.

Update + Clarification

Outlook Safe Mode has not helped.

BitDefender is the suspected cause (only change), but BitDefender has not caused this problem on other identical machines and the problem persists after uninstall so it must have changed something in Outlook if it is the cause.

Solution

I don't think you can blame Exchange for this one - it sounds very much like Outlook at fault (though it probably has a bit of help from a dodgy add-in).

The fact you say it isn't a problem on other computers without BitDefender on them just gives me more reason to think it's BitDefender at fault here.

Try starting Outlook in safe mode (outlook.exe /safe) and try sending an email to an "unknown" address to see if it works. If it does, disable all installed add-ins and enable them one by one until the problem reappears.

Comments

[Graphain]

Thanks and I'm inclined to agree, I don't think it's reaching Exchange. The first thing I tried on Outlook after removing BitDefender was safe-mode (should have mentioned that). Just to clarify, BitDefender is currently NOT on the PC that is not working and IS on the PCs that are. BitDefender WAS on the broken PC and is the suspected culprit (only recent change) but I cannot find what it changed within Outlook. I'll try an Outlook reinstall and see where that gets me.