POP (Post Office Protocol, with the latest version being POP3) and IMAP (Internet Message Access Protocol) are two of the very first protocols developed on the consumer Internet that allowed for email clients - like Outlook, Thunderbird and others - to retrieve mail from a mail server. The ports typically used for POP are TCP ports 110 and 995, and for IMAP are TCP ports 143 and 993, for insecure and secure sessions respectively. They were each good at doing different things, like reflecting the state of an email back to the server (whether it was read, flagged, or marked as junk), or for preserving a copy of the message on a local machine for easy offline access. The latest version of POP, POP3, can be used with or without an SMTP. This does not affect which port you can use with Mailgun. Mailgun doesn’t host mailboxes, so these aren’t protocols we support.