We have recently installed TMG, and use it as a proxy server + firewall. Although, i have enabled Outlook at TMG client settings, for one specific workstation, POP3 + SMTP traffic is not allowed. More specific, after checking logs, I noticed that after initiation of successful connection to Outlook, then I take denial with the following "FWX_E_TCP_NOT_SYN_PACKET_DROPPED".
We have enabled Outlook, as the suggested link, that you provided, guides.
Also, our internal network is populated through TMG, and the user's PC IP is in that range.
The weird is that this happens only for a specific user, who is authenticated by the TMG Firewall client, and access Internet with no prob. When it comes to send/receive emails using Outlook, he gets error connections.