Your best bet is to go to the email provider's websites, and see

1. if the first account can be set to forward to the second account, or
2. if the second account can collect the messages from the first account on your behalf.

Option 2 assumes you are the owner (or at least an authorised user) of both accounts.

But I feel I may not have fully understood your request. It may help to name the accounts, say "1" and "2", or "A" and "B".

You say "I also do not want to add the account to my account details" - but which one is "the account" being dicussed here?

Hi @zenos. Thanks for your assistance. I think I need to clarify my point a little further as it isn't specific enough.

So I currently have 2 Thunderbird accounts and need to basically merge them so I want to take the current address being ABC@123.com and make sure that any emails that come to that address get redirected to ABC@321.com

The reason being is that some clients use the one and others use the other so I want to avoid having to look through 2 separate inboxes.

If you want to receive messages addressed to either of these addresses then you need to either add both accounts to Thunderbird, or use one of the two ploys suggested before to make all your incoming messages appear in one Inbox.

I have 8 or so email addresses. I use View|Folders|Unread to highlight all my unread messages, regardless of their location. I also use a Saved Search for collating specific work-related unread messages, the advantage being that it is more selective about which folders it examines for you.

One snag with merging two (or more) accounts into one location is that it becomes all too easy to reply or start new messages using an inappropriate identity.

I like having my messages in different accounts; it help keeps work, business and pleasure separate. The trick is to become good at finding new messages, rather than trying to corral them into one place.

You might also consider an add-on such as QuickFolders, which can be used to make regularly-used folders more visible and accessible.