From the Microsoft's
If you are having a problem connecting to an ISP, try clearing the Use
software compression and/or Use IP header compression check boxes.
If you re having a problem connecting to an ISP, try lowering the baud
rate under Connection Preferences in Device Properties to the next lowest
setting, until you get connected.
Known issue with some mail clients for CE (and other palm tops)
Users attempting to send email will get errors saying "Access Denied" from the
SMTP server smtp.nwnexus.com. In order to get around this, when specifying
a return address in the email program the address must be surrounded by
left-arrow bracket and right-arrow bracket:
Return Address: <firstname.lastname@example.org>
By default, some mailers do not include the brackets, and mail is rejected by
our outgoing mail server.
For more information on WinCE, here's some helpful sites:
WinCE Support Page
Craig Peacock's Windows CE