Hi Fl iMac,
Thanks for using the Apple Support Communities. Sorry to hear you are having these issues with your Mail account in El Capitan. If you suspect that it may be an incorrect port issue, it is possible to change the port configuration for both the incoming and outgoing mail servers, just not in the same place:
- For the incoming server, you would go to Mail > Preferences > Accounts, select the account in question on the left hand side, then select the Advanced tab to the right; it should say Port: and allow you to select the Authentication.
- For the outgoing server, you would likewise go to Mail > Preferences > Accounts and select your account on the left. Then, in the main Account Information tab, where it says Outgoing Mail Server (SMTP), you would click on the drop down and select Edit SMTP Server List... In the window that opens, you would select the correct outgoing server from the list at the top, then select the Advanced tab below. Again, you should see an option to set the Port: and Authentication.
If editing the ports does not resolve the issue and/or you are still having other Mail issues, you may also find this article helpful:
If you can’t send or receive email on your Mac - Apple Support
Regards