SMTP Test Tool is web-based SMTP test tool so you can run diagnostic sessions on SMTP servers over the web, rather than having to use telnet and issue manual commands, which can be cumbersome.

If you’re having difficulty getting emails delivered by your SMTP server, it might not be readily apparent what the issue is, and you might find yourself writing test code to figure it out. With this tool, you can easily see what’s going wrong with an SMTP server. Perhaps your login credentials are failing. Perhaps the SMTP server has run out of disk space. Perhaps the SMTP account is throttling you because of pre-established sending limits. By looking at the exact SMTP server responses, you can easily troubleshoot an issue.

https://www.gmass.co/smtp-test