If this is a POP account, you can highlight the messages, right-click and 'copy to' the other domain. If this is an IMAP account, it's dicey, as network interaction is needed. I suggest either doing this a small number at a time, or setting up a filter to do this and manually run it. You need to do this carefully to avoid having the messages appear to have been copied when, in fact, the content is found later to be missing. A simpler approach to keep the messages would be to create a local folder and copy them there. They wouldn't be within an account, but you would at least have them.

re :y@domain2.com receives emails from multiple domains in it's inbox,

Not sure how y@domain2.com has been set up. Maybe this has been set up on the server, so all emails are already redirected in to that email address. which means in Thunderbird it can download everything already on server. Then in Thunderbird you set up the various email addresses as identities of that one mail account. Q: Do you have all various emails being received into the one place on server which you then download?

Maybe this account is a POP account - maybe it's an iIMAP account which can only display whatever is stored on server. Q: what type of account is y@domain2.com - Pop or IMAP?

IF POP: Maybe you set up all the individual email addresses as POP accounts in Thunderbird, but set up each one as a deferred account - commonly known as 'Global Inbox' which uses the 'Local Folders' account as a common Inbox etc for all emails - so you do not see any individual pop account.

Q: Did you set up all those various domain email acounts as POP accounts as deferred accounts using 'Global Inbox (Local Folders)' ?

Your answers to the above determine what is possible. If you have a load of mail account all going to one email account on server and then downloading it then perhaps you do the same for the x@domain1.com account with just one email address attached to it.

Q:But what type of account have you currently set up for x@domain1.com ?

If y@domain2.com is an IMAP account, you cannot simply add an mbox file. IMAP accounts can only display whatever is on server. IMAP accounts cannot be considered as a truelly independent copy on your computer because everything synchronises with server folders. As the mbox file is a folder with emails that does not exist on server, it will be auto removed on startup.

To get emails into an imap account means you have to it by one of two methods: 1. Get the domain1 email address to be auto redirected from one server to another server for domain2 email address - this is done directly via the account - logged on via a browser. Then they will get auto downloaded into correct domain2 account in Thunderbird.

2. Manually upload copied emails in small batches - select a batch of emails from one Inbox to highlight - right click and select 'Copy to' and choose the relevant folder on Imap domain2 account. You are not just copying from one folder to another - any emails you copy into an IMAP account folder will get uploaded to the server in order for it to be displayed in an imap account. There will be a daily limit on how much you can upload onto server, so would need to check that out. It might take you a while and you also need to consider how much quota it will use as all servers have a quota limit.

Another option which you may consider as it is quick, offers you a single Inbox to view all emails received into all Inboxes and does not involve uploading nor copying anything.

Use 'Unified Folders': View > Folders > Unified

Then you will have a virtual single 'Inbox' folder which will display all emails from all Inboxes in one folder. That virtual Unified Inbox will have sub folders - they will be the various separate Inboxes which can choose to see if required.

Thank you both! The "Copy To"/"Move To" option for IMAP emails is what I was looking for, it looks like this can fulfill the task.

I would advise using the 'Copy-to' because it's less risky. If something goes wrong with a 'Move-to' upload then you may lose an email if it's been already moved out of one folder, but gets lost in an upload to server. But if it's just performing a 'Copy-to' then the original will still be in original folder and you can repeat the operation.

Good point, I'll use that one then. Thanks again!