Personally I use webmail (RoundCube is my fave [edit June 2018: I am really liking RainLoop and using that more these days] ) almost exclusively. But if you are using an email program installed on your computer, you will need to set up the connection ports.
Below are the connection ports to use on Cpanel hosted accounts, please note that Plesk or Windows may be different:
993 (for SSL/TLS) (*Recommended)
143 (for PlainText/StartTLS)
995 (for SSL/TLS) (*Recommended)
110 (for PlainText/StartTLS)
465 (for SSL/TLS) (*Recommended)
25 (for PlainText/StartTLS)
587 (for PlainText/StartTLS)
* Recommended options provide the best security and performance.
Note: When using SSL/TLS connection (recommended) be sure to choose "accept all certificates" option. That process is different for depending on which mail program is being used.
If you continue to receive "Certificate" related errors, you may need to add SSL to your domain. We HIGHLY recommend this for a variety of reasons, including new SEO rules in Google search results. To add SSL (HTTPS) to your domain, see here or contact us for pricing: https://www.hostned.com/web-hosting/hosting-options/