Ask HN: How to test Email Clients?

Hello all,

I want to test email clients in my project. I could come up with test cases like:

1. Send mail in outlook, delete mail in mobile app, check in outlook it shouldn't be there etc.

2. The supported attachments

3. Check sender has message limits. and go on.

These are simple test cases that I could come up with, but what are other test cases that I "must" test while testing email clients? That can be in :

1. Security

2. Usability

domain.

Any ideas will be really helpful.