The default action: If you are reading emails in the new mail account and then click on 'Write' or 'Reply', Thunderbird assumes you want to use the email address of the account you are viewing and so will insert the new email address.

If you are reading emails in the old mail account and then click on 'Write' or 'Reply', Thunderbird assumes you want to use the email address of the account you are viewing and so will insert the old email address.

The mail account listed at the top of the Folder Pane is usually the default mail account. You can set the new mail account to be the default.

• Right click on new mail account name in Folder Pane and select 'Settings'
• The 'Account Settings' window opens and the new mail account name should be selected. If not make sure the new mail account name is selected.
• click on 'Account Actions'
• Select 'Set as Default'
• The default mail account will get moved to the top of the list.
• Click on 'OK'

see image below as guide.

If you inadvertantly use the old email address to send, but would like future 'Reply' emails to be sent to the new email address then you could set a 'Reply-to' email address for that account. This could also be a good way of gradually getting all new emails into the new mail account.

• Right click on old mail account name in Folder Pane and select 'Settings'
• The 'Account Settings' window opens and the old mail account name should be selected.
• On the right side locate 'Reply-to Address' and type in the new email address
• click on 'OK'

There is an addon which may also help. I do not use it, so cannot comment on it, but it may be worth checking it out. It allows you to set a default To address for newly composed messages based on which folder you are working in. So you could set up all the old mail account folders to use the new mail account email address.

• https://addons.thunderbird.net/en-us/thunderbird/addon/folder-account/

Thanks for your prompt and clear response. I did change the 'reply to' address which should help. And I downloaded the add-on, although I haven't figured it out yet. However, your response gave me a couple of ideas:

1. Under 'account settings' for my old email address, could I change the "email address" (not account name) to my new email? Or would that mess things up? When I inadvertently send an email from my old address, I would like the 'from' field to show my new address, so the old address doesn't go into the receiver's address folder. I know I've had that problem when friends change their email address but then email me from the old address. 2. What if I changed the SMTP server address to something that wouldn't work at all? Then, TB would not be able to send email from that address, which would alert me to the fact that I'm sending from the old address when I should be sending from the new one.

Thanks again for your help.

Is your old address and new address with the same mail provider (I hope not)

No, they are different.

then go into account settings (the entry above the server settings for the account and change the SMTP server to be used from your old account to your new.

Your new provider should refuse to send mail for the old account, so if you inadvertently try and send mail from your old account you will get a send error. Note that you actually have to make some setting changed a gmail if they are your provider. changing the email address as you have suggested would force your relies to show a from that is your new address and would also allow the mail to pass through the new SMTP server without error.

You might also change the your name to Note the new email address for and then your name. As this would only show on mail from your "old" account it would be a good prompt to those emailing you to update there address book. alternatively, you could remove it altogether. That would see the mail go out with only the new email appearing in the from for the recipient. (Not display name as part of the from address (unless they use Thunderbird which uses address book display names if they are in the address book.)

Hi Matt,

Thanks for your reply. Your suggestion is very clever and I think will solve my problem! However, I don't understand this part:

"Note that you actually have to make some setting changed a gmail if they are your provider. changing the email address as you have suggested would force your relies to show a from that is your new address and would also allow the mail to pass through the new SMTP server without error."

Could you clarify? Thanks for your help.