Gmail supports two protocols POP3 and IMAP. But Airmail definitely recommends IMAP over POP3 protocol for better support and performance. Following the screenshot below, which shows you in the blue box that you’ll need to have IMAP option enabled in your Gmail Settings in order to get IMAP working with Airmail.
In the screenshot you could also see “Folder Size Limits” you could set the same option as the above screenshot in order to get a better syncing speed and better search result. But it’s not recommended to keep it Limited since because it couldn’t able to search what you’re looking for in Airmail which is exactly same as web-client.
In the above screenshot you’ll need to tick all options to get mapped in Airmail if you do not have those ticked Airmail will not work properly with IMAP they may be go unstable or unnoticed. Strongly we recommend you to keep those option checked.