I would advise you to create a new existing mail account for the new email address. The settings are referenced to various preferences and it is not worth messing up. Thus having the two mail accounts, but read on as there other options depending upon what you are trying to achieve.

• Menu app icon > New > Exisitng mail account

Usually, if the email address is the same, then it is ok to swap just the server settings - not the email address itself. This may occur if you had a hosted website and needed to change hosts, but before changing server settings, you would need to make sure emails were secure first as explained below.

If 'xxx' is an imap mail account then it is not advisable to change settings because imap accounts only show what is on server for that 'xxx' email address. This means, if you change the settings to point to another mail account 'yyy' then all emails in the 'xxx' account folders will disappear.

So, if using imap for 'xxx' then I would advise you to do the following if there is an intention to stop using 'xxx' email address.

You would need to synchronise all 'xxx' folders to download full copies and then go into offline mode. If you can read emails in offline mode then you have downloaded full copies to folders. Now you can create folders in the 'Local Folders' mail account and put copies of imap 'xxx' emails into the 'Local Folders' mail account folders. This means those copied emails are now independent of the server and if you can still read them in 'Local Folders' account, you have good copies. I say copy them because if something fails you still have the original to try again. I also advise you doing this in offline mode because it stop all folders synchronizing whilst you are getting a lot of copies of emails into 'Local Folders'.

Info on synchronising.

• https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage

If intention is to stop using 'xxx' and you have saved copies of emails as described, you can delete emails in 'xxx' account and emails will also get deleted off the server. Then unsubscribe from seeing folders before finally removing the imap account.

Another option:

You could create a new 'yyy' identity for the 'xxx' mail account. So all emails are coming into the same Inbox like a catch all account. This maybe a preferable method, so worth reading up on information. Note :where it says To create an identity, click Tools > Account Settings…, that referes to 'Tools' on the 'Menu bar ' toolbar. If using the 'Menu app icon' on the 'Mail Toolbar' toolbar then select 'Options' > Account Settings' OR alternatively, right click on mail account name folder in Folder Pane and select 'Settings'. All three ways will access the 'Account Settings' window.

• https://support.mozilla.org/en-US/kb/using-identities

If [email protected] is a pop account, then usually all emails will have been fully downloaded, so you should have full copies. You could copy them into 'Local folders' account, if you need to keep emails and want to remove the pop account. But as I understand it, you are not getting rid of the 'xxx' email address as you may still get emails, just adding another which you may intend to use more frequently. In which case either create a new existing mail account for 'yyy' or consider setting up an additional identity for the 'xxx' account.

You could also set the pop 'xxx' account up with an additional 'yyy' identity and then swap the 'default' identity, so 'yyy' becomes the main account.

Another option:

In 'xxx' webmail account accessed by a browser you may have the option to forward all 'xxx' emails to the 'yyy' server and then in Thunderbird set up a new 'yyy' account which would download everything from yyy server to yyy account including all the xxx emails.

Thanks for your very detailed answer. Meanwhile I found out the article regarding creating a global local mailbox where both accounts xxx and yyy will be in the same folder: https://support.mozilla.org/en-US/kb/unify-your-pop-email-accounts-global-inbox I backed up my existing xxx account using Mozbackup, followed directions in the article, restored xxx from backup and had to to copy xxx profile to local profile. In the end it looked like it worked. I tested all this al my second Win 10 pc, now will do the same on the main computer. Thanks again for your help. Regards, Yuri

re :I backed up my existing xxx account using Mozbackup,

It is not recommended - even the person who created the product says there are known bugs. MozBackup has not been maintained in years. Just exit Thunderbird, access the User Account\Appdata\Roaming folder and copy the 'Thunderbird' folder to an external drive as it will backup everything.

Global Inbox for two separate pop mail accounts. You cannot use this for imap accounts. Basically you will have created two pop mail accounts and direct them to use the 'Local Folders' Inbox. It means you do not see any mail accounts in the Folder Pane, only 'Local Folders' mail account, although both accounts will be visible in the 'Account Settings' window.

Important: Remember to exit Thunderbird and restart before downloading mail into any account whose Inbox/Global Inbox setting you've changed. If you do not exit and restart, messages might continue to download into their "old" locations (e.g., into the individual account Inbox rather than the Global Inbox). Slightly older info but it's all relevant.

• http://kb.mozillazine.org/Global_Inbox

I didn't know about bugs in Mozbackup. Will backup Thunderbird folder from now on. Thanks again for your help. I think I'll get rid of the Global inbox, and just add the second account to Thunderbird.