There are many issues/bugs with different accounts, including Gmail and iCloud.

Rebooting doesn't solve anything. Uninstall and start anew, and then, maybe, it works, for the time being.

When composing an email, Airmail can respond very slowly to keystrokes. Customer service refuses to acknowledge this problem, responding that it's "not real" - which is a very odd response, to simply refuse to acknowledge the customer's experience.

Paid version only.

On some IMAP accounts, there are a number of serious sync problems, which otherwise work fine with Apple Mail.