List of free email-testing stuff for developerππ½
If you're looking for all programming tools around email-testing (not just free). Check this page List of email-testing Programming Tools

Imitate Email
Enable software teams of all sizes to easily test email flows, whether writing your own software or using a third party, and across in-house and external users.
Tag: email-testing
Free for solo developers with 450 test emails a month and access to all features

Mailtrap
Email Sandbox Service Capture SMTP traffic from staging and dev environments Automate test flows and scenarios with flexible API Analyze email content for spam score and validate HTML/CSS
Tag: email-testing
For new users who want to evaluate Mailtrap or intend to apply it on a personal side project. 500 Email test / month

Testmail App
Test new user signups, transactional emails, spam scores, and more. Get started on our free forever plan with unlimited email addresses and mailboxes.
Tag: email-testing
100 emails/month Unlimited users, email addresses, and mailboxes 1 day retention One random namespace
100 emails/month 1 mailbox 3 users Change your code inside Pacomail and see the result instantly.