DVMadison said

I'm just confirming my understanding - because the underlying issue is important. Recently, I was mistakenly sent an email containing and attachment with information I'm not supposed to have. I need to confirm with the sender that I permanently deleted the attachment, and I think I have - but I want to make sure I reeaaaallly have deleted it. There are legal implications if I failed to actually delete it. I'm using Thunderbird at work with an IMAP server (and Outlook as the underlying email service). I deleted the email in my Thunderbird client on my desktop, which did not delete the email off the IMAP server. I logged into my account on the server and permanently deleted the email. My understanding is that attachments are stored inline with the email, so I believe that should do the trick. Can you tell me if I have actually removed any traces of this attachment from my computer and the server?

DVMadison said

…I deleted the email in my Thunderbird client on my desktop, which did not delete the email off the IMAP server…

I don't understand how this can happen, if you're using Thunderbird. Some IMAP clients on phones would appear to operate like this.

Otherwise, you're right. The attachment is stored with the message; delete the message and the attachment is gone too. To fully remove it you should compact your folder too. Until you do this, the message and its attachment may still be in place but hidden.

You're an innocent party here. I'd have thought the person who mistakenly sent it to you would be the one to carry the can. You can't prove that you don't have a copy, somewhere. "Absence of evidence…".

Yeah, I don't understand either. the fact that it didn't work the way I was expecting is a source of my worry. I deleted it on TB, and then logged into my account on the server directly, and it was still there. hence....

As far as my culpability goes - yeah, I didn't created the problem, but the thing I was sent was the type of info I have to periodically legally certify does not exist on my computer, no matter how I came by having it, I need to know that I actually deleted it.

The issue is Thunderbird normally only send an expunge commands to the server when a folder is compacted. There is a user option in server settings to expunge the inbox on exit, and this goes a long way to getting the job done. But for a full expunge then you delete, you need to use the config editor to set mail.imap_expunge_after_delete to true

This might cause some overhead and make things slow. use at your own discretion.