Anyone who has had to deal with HTML emails on a technical level has probably reached the point where they wanted to quit their job or just set fire to all the mail clients due to their inconsistent implementations. But HTML emails are not just a source of frustration, they can also be a serious security risk.

An email and SMTP testing tool with API for developers - GitHub - axllent/mailpit: An email and SMTP testing tool with API for developers

Betterbird. Simply better. A soft fork of Mozilla Thunderbird

Mailarchiver, OS