Since iPhone OS 2.0, many companies have been introducing the iPhone to their mobile Exchange client list. However, not all companies run their Outlook Web Access (OWA) server on port 443. In these cases, an iPhone can in fact use ActiveSync on an alternate port here's how:
1. Set your Exchange settings on the iPhone as normal.
2. Put a colon with the port number after the server name. It should look something like mailserver.company.com:4645
3. Enjoy your email.
(Updated 2015-01-01 to include screenshot)