The problem is that both addresses do share a single account with your provider. With that setup, you each will always be able to see the other's e-mail.

I suggest changing that, separating the two addresses into two actual separate accounts as that would be the easiest solution that will solve your issue forever, no matter what e-mail program you use.

If you don't want to do that, then to get Thunderbird to do what you want, you will need to use Message Filters. Organize Your Messages by Using Filters

You set up a filter that has your address for "To", and have it send the messages to one folder. Then do the same for your wife's address, but a different folder.

Are you sure these are two distinct accounts? It sounds rather as if the second address is set up as an alias of the first address and so they are effectively the same email account. At face value, your penultimate sentence: "Both email address share one account at the address provider" confirms this, but I am wary of how people use words like "account". Did you mean "email account", or did you mean your account with your ISP or email provider?

I have two addresses with gmx.co.uk, one being the xenos address I use on this forum, and the other using my real name. Just as you describe, all messages addressed to either account pop up in both inboxes, because both are, under the hood, the same account.

The "proper" answer is to get the second address set up as an email account in its own right. You'll need to go to your mail provider or his website to arrange this.

Otherwise, you can probably bodge this in Thunderbird by using a Message Filter to divert messages addressed to one address into a separate folder. But I wouldn't do it that way. I wouldn't want the risk of anyone else deleting my messages, whether by accident or design. And there are occasions when arranging a surprise gift or outing where I wouldn't necessarily want my wife to see my messages.

https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters

I call the use of a Message Filter a bodge because the filters can only work on what has already been downloaded. You can't easily put a message back if you find it's one you didn't want to see.

If you can't get your email provider to offer these as two distinct email accounts then the best bodge would be to set up filters on the server so that the messages were separated into their own folders before you attempt to download them. But you'd still be able to see one another's messages, and in suggesting this, I am assuming you are using IMAP, because the alternative POP can only deliver the so-called Inbox, and filtering at the server would render the filtered messages invisible to any email client.

Thank you for your answers. Both were very helpful and clarified the issue for me. Both addresses do belong to one email account as you both correctly wrote. I'll give the filters a try just to see how it goes and I'll set up a new account if I'm not happy with it.

Thanks again for your clear thorough answers.

All the best.