I am using Windows Vista Business with Outlook 2007 and a Cisco VPN client to access my corporate Exchange Server. Everything worked fine until I went on travel last week and connected to a hotel network (both cable / wifi). Upon my return, I could no longer send email with attachments. I can connect via VPN. Send / Receive email. Ping endpoints. Access Sharepoint, etc. Only when I try to send an email with a large file attachment, 100KB +, does the transmission hang.
When I use my Verizon Broadband card , which establishes a PPP connection, the email works just fine over the VPN. I also installed a PCMCIA ethernet adapter and had the same problem, so it is not my internal NIC.
I am using a Comcast Business class ISP connection so bandwidth is not an issue. It appears to be a problem with Ethernet in general.