Ranked in these QuestionsQuestion Ranking
Pro Open source
Mailtrain is a free and open source software released under the GPL license.
Pro Supports several providers
Mailtrain supports any provider that you may want to use. For example, you can use Amazon SES, SendGrid, Mailgun, etc.
You can basically use any provider that supports SMTP.
Con Virtually no documentation
The official documentation only covers installation. Everything else on how to use the software is missing.