SYMPTOMS
Sending or receiving email fails on the DATA command.
CAUSE
This problem is most likely an issue with the MTU settings of the mail client TCP/IP settings. Windows defaults to a higher frame size than is supported by many routers.
RESOLUTION
The solution is to change the Maximum Transmission Unit (MTU) settings for the IP stack.
This is not specifically a MailEnable problem, but a generic problem/configuration issue with Windows NT when being accessed over hybrid routers.
Use the 'Diagnoses and treatment of black hole routers' article at this link: http://support.microsoft.com/default.aspx?scid=kb%3ben-us%3b159211
REFERENCES
How to find max packet size for DSL, Cable, etc: http://www.tek-tips.com/gfaqs.cfm/lev2/5/lev3/60/pid/581/fid/1995
Change the Maximum Transmission Unit (MTU) Size: http://www.winguides.com/registry/display.php/280/
The MTU Fix for Cable & DSL.: http://www.infinisource.com/techfiles/maxmtu.html