Q: An error occurred while moving messages to mailbox “(null)”.

This seems to be the resolution of the problem - worked for me too.

I found a way to fix this.  I assume you're using an IMAP account.  If so, you should be able to expand the mail account and see the folders that exist on the server.  Find the one you want to use for Trash, select it, then go to the "Mailbox" menu and find "Use This Mailbox As..." and select "Trash Mailbox".  You can do the same for sent messages, junk, etc.  Once I did this, the message stopped appearing.

It also help me, thanks for sharing

I know it's late to be asking this question, but when I select the mailbox and then Mailbox/UseThisMailboxAs   all the mailboxes are greyed out and I can't pick any of them.

This worked great for me and fixed the problem!!!  Thank you @tbiske

This worked for me! I also had noticed a sent message disappearing. I fixed this the same way by selecting the Drafts, Sent, Junk and Trash mailboxes that appeared in the "Inbox" as lists under the malfunctioning mailboxes, and individually assigning them properly in the "Use this mailbox" pulldown menu.

Thanks!

I am in the same boat.  The menu is greyed out. Anyone?

Thanks for your help man. That did the trick!

It did for me too until I picked the correct trash folder first and then went back up into the "use this mailbox as" folder where it became ungreyed. Make sure you select the proper folder where the trash is going to be.

Ah... Now I see.  Bingo. Thank you!

Thanks for the solution, tbiske. Worked for me after having to reinstall IMAP accounts in Mail.

Have seen this error from time to time and tried the different solutions suggested here. Then looked around and found a trash folder 'On this Mac' which shouldn't be there, as all accounts I use are IMAP, and I want trash to go to folders of each of the accounts e.g. Gmail.

I could see that some of the emails that refused to be 'deleted' (moved to trash) was in this general 'On this Mac' folder.

I deleted the 'On this Mac' folder and it seems to work. Btw. I found similar extra local folders for spam and send mail too.

I'll be back if this solution doesn't work.

I'm sure now this is a bug. And sorry, it's not Yosemite but El Capitan. Mail functions correctly on my iPad, same IMAP accounts. Tried the advise to rebuild the mailbox, and it doesn't work, so I'm going to try to remove all accounts and ad them again.

Removing map accounts and setting them up didn't solve the issue, but right now there was an update to El Capitan and Mail.app (9.2). Hope this works.

I started getting this error today "An error occurred while moving messages to mailbox “(null)”.

But when I try to:  "Use This Mail Box As"  > the options are all greyed out.

For some reason two of my mail accounts are also in the TRASH with strange Black Squares as their folder icons.

Any one else?

Thanks for any advice.

Cheers

A.

I've set up Mail on my Mac (OS X) iPad and iPhone. No problems with my Gmail account. And no problems with iOS.

Last time I set up an IMAP account, say mail.npsonder.com I had this problem with my Archive folder. Also in Mail (on Mac) all folders were found under Inbox instead of the normal way in the left panel under e.g. Npsonder.

To resolve the problem I 'just' had to enter the right names of the IMAP account instead of the generic mail.npsonder.com that worked properly for most actions. The actual name was like mail14.ispname.com, and everything works right now.