Currently the email domain is stripped from the username:
https://github.com/exolutionza/lilmail/blob/93b9f9344f3a92980545f1202be57e4908c4c6cf/handlers/web/auth.go#L61

however a lot of mail servers expect the full address, making it impossible to login.

I've switched to a different web client, but I thought I'd mention it just in case.