This has happened to me many times over the past several years with eM Client… when I delete a message in the “All Inboxes” view using the “delete” key and then (quickly) undo it with CTRL+Z, the message returns to the view only to disappear a second later. When I attempt to find the message, thinking that it is back in the trash, I can’t find it anywhere. It’s not in trash, all mail, etc. I can also confirm this by logging into gmail.com. The message is nowhere to be found.
eM Client Version 9.1.2109 (9967b93)
Email Provider: Gmail / G-Suite Business
OS: Windows 11
I cannot replicate that problem with the same eM Client version and a Gmail account.
Might be a problem with the account in eM Client or the program installation somewhere.
I would suggest to first backup eM Client (if you don’t regularly already) via “Menu / Backup”. You can see when the backup is completed via “Menu / Operations”.
Then go to “Menu / Accounts” and “Remove and Re-add” your Gmail account. When you remove and re-add your account eM Client will take time to re-read your messages again “until they are re-cached”.
If makes no difference removing and readding your account, then restore your backup via “Menu / File Restore”. Next try uninstalling eM Client and “Don’t delete the database on uninstall when asked”. Once uninstalled, re-download V9.1.2109 from the version history page and reinstall the program and see if that fixes it.