Sorry to hear that. But, there's a couple things to try here. Let’s roll up our sleeves and hopefully fix this for you.

1. Try compacting folders

• Go to your email account > right-click a folder like Inbox or Trash > Compact. This cleans up “deleted” space and can improve speed - especially if it's been some time since this action was performed.

2. What kind of email account is this?

• If it’s an IMAP account (like Gmail, Yahoo, Proton via Bridge, etc.), Thunderbird has to sync the deletion with the server. If the server is slow or there's a lot of back-and-forth, then that slow communication can cause delays.

3. Are you using Proton Mail with Proton Bridge?

• Proton Bridge can sometimes slow down when handling bulk actions like deleting multiple messages, especially if Proton's under heavy load

You could also just got to File -> Compact Folders and do all of them at once. But, where's the fun in that?

Did the above message help your issue?

NI not answered. Closing question as solved

I am having the same problem...sometimes (as in all too often) when I get notifications from my calendar, sometimes when I select an e-mail reply for view, I get the "Not Responding" message and must wait and wait for the program to resume working. Not sure if I have an add-on that is causing this or some other internal/local problem. Suggestions (Compacting Folders doesn't affect my problem) please.