[feature request] add ability to check the sender account

[piroor]

Original: https://addons.thunderbird.net/ja/thunderbird/addon/flex-confirm-mail/reviews/1163262/

When there are multiple SMTP servers, it should be available to check which SMTP server is going to be used.

[sphh]

That's a great feature I am looking forward to be implemented. Maybe it should not check for the SMTP server, but for the identity, because often different identities use the same SMPT server. Thanks.