My users are a mix of Outlook 2007 and 2010 and we have an onsite Exchange 2010 SP3 environment. They can receive email and send new messages to the vendor's email address, but when anyone reply's to a message from that vendor, Outlook times-out. They have to kill Outlook and restart it. Again, this only happens when replying to an email from this specific vendor. I've confirmed this by having this vendor send me just a plain test message with no attachments and I received the email promptly. When I reply to the message, Outlook hangs and eventually times-out. I tried playing with the Outlook Account Settings/Server Timeout settings but that didn't help.
No issues have occurred if we use OWA and reply to the same email address.
Not sure if this helps, but the vendor email address ends with .ca