Just noticed - even Mozillas own Firefox for Android has an undo popup when you close a tab via swiping. And that is just closing a tab, not possibly deleting a very important mail.
I’d also love an “Undo” feature after swiping to delete or archive.
It happens to me several times a week! Swipe one way or another by mistake and loose an email. Sometimes I remember which one it was and can go fish it out from the archive or trash folder, most of the times I have no idea what I lost there, which is a serious usability and productivity issue.
So a thousand times yes for having an Undo button appearing after a swipe move!
I think you need to have a look at your settings. It is possible to set various actions for swipe gestures and it is now possible to set delete confirmation. It is not quite an undo button but it may resolve your problem.
I have my Thunderbird on Android set to Italian but it will be something like:
Settings
General settings
Interaction
Confirm action
Hope this helps. Personally I don't like swipe to delete but that is a personal choice and selectable.