For example, if you're using an SSL VPN proxy, it may not be designed to support native e-mail client applications...
at all. Instead, it may be necessary to use a web portal interface to reach your e-mail server through the SSL VPN gateway. In this case there is no e-mail client reconfiguration necessary -- you simply use your browser as your e-mail client. The SSL VPN gateway is then configured to send e-mail to the port your e-mail server expects -- that is, the gateway can send plain old POP3 (110) and IMAP (143), or you can send POP3S (995) and IMAPS (993).
If you're using an SSL VPN that port-forwards native protocols over SSL, you'll probably need to reconfigure your e-mail client to send outbound traffic to localhost instead of the destination e-mail server. The SSL VPN agent running on the local host will intercept e-mail traffic and forward it over the SSL tunnel. Upon receipt, the SSL VPN gateway will forward that e-mail traffic on towards the e-mail server. In this case, whatever port the e-mail client sends traffic through will be the port the e-mail server receives traffic on.
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