What you need is to use Thunderbird's Identities feature. See Using Identities for details, then create a new identity in Thunderbird that uses your other email address. To send replies via Gmail's outgoing server, but using that other email address, just add that address to your Gmail account and enable sending using it.

Hi, Thank you so much for your help. I think I understand what you explain to me but I can't find anything about "identities" in Thuderbird 91.2.0 (64 bits)...

I found something in the account parameters in Thunderbird but it remains unclear to me.

You'll have to be more detailed and specific than that. If you've read and tried following the article I linked to, specify what it is you tried to do and what is not clear. Help us help you.

Hi,

Thanks. I'll try to be more explicit. (English isn't my native tongue.)

My main e-mail address is MainAddress@gmail.com I have another address with gmail : "address2@gmail.com" A third address3@xyz.com A fourth with microsoft e-mail service. etc.

I set on the gmail website an import of all these accounts and a forwarding of new messages of these accounts to this Mainaddress@gmail.com.

Now in Thunderbird, I only use the MainAddress@gmail.com inbox anymore because all other messages go there. (but in Thunderbird, I haven't deleted the other boxes yet)

In the mailbox of my mainaddress@gmail.com in Thunderbird, messages that were addressed to (for example) address3@xyz.fr are displayed with this recipient and not mainaddress@gmail.com.

I just want to be able to reply (very simply and in Thunderbird) to my emails as before with the sender address3@xyz.fr even if I do it from my mainaddress@gmail.com mailbox (I think it works on the gmail website but not in Thunderbird.)

Thank you for your help.