<p>A Sprint cellular modem plugs into a laptop - often the PCMCIA slot. To connect, the user opens a program called the Connection Manager. This program has a menu option to display the serial number and phone number of the modem that's currently inserted.</p>

<p>Does that imply that the serial/phone number of the modem could be available to other programs running on the system? (Note: let's assume this is Windows XP.)</p>

<p>Specifically, could a company's VPN software be configured to pass along information about which modem is being used to connect?</p>

<p>Finally, is there existing VPN software that already does this, or would it have to be custom-programmed?</p>

You could send the mail asynchronous. That way the timeout should not interrupt your sending.
This article should help you get started with that:
Sending Emails Asynchronously in C#
.
There is another approach here:
http://www.vikramlakhotia.com/Sending_Email_asynchronously_in_AspNet_20.aspx
And off course there are several commercial clients available, but the only one that i have tried and can recommend is
http://www.aspnetemail.com/