- https://github.com/atharrison/go-mta - smtp-source -c -d -m1 -s1 -v -l1024 127.0.0.1:9999
- Mailozaurr (powershell module based on... .NET libraries)
- mimecast/robin Debug and development tool for MTA architects! Robin is a highly configurable SMTP client for testing and debugging SMTP servers. https://github.com/mimecast/robin
- smtp-starttls-ciphers BASH script to test the MTA ciphers supported based on the NCSC security guidelines for TLS (Dutch) https://github.com/jbouwh/smtp-starttls-ciphers
- Granikos/SMTPSimulator https://github.com/Granikos/SMTPSimulator