When using an alias (firstname.lastname@example.org) it shows apparently as that when I compose. On the receiver end, it shows as my main email (eg. email@example.com). When the receiver replies, it is sent to the alias address. I would like not to disclose the correct email to the receiver. I have done this using Thunderbird and identities, in the case of eM Client, it seems to insert a reply-to in the sent email instead of using the alias. This is also verified by looking at the source of the email.
I have had that circumstance for many years. Prior to using eMClient. Using MS LiveMail and other MS email clients for a good 15 years or more. I use an alias for all the usual filtering reasons, etc. and try not to publish the real address but, exactly as you describe, it gets put on to the emails which leaks it. Apparently you found some way using Thunderbird to preclude this leak. I’ve never figured that out.
Yes, that is correct.
The alias is dependent on your email provider to allow sending from an address other than the one you use to login to their server.
In most cases they do not allow that, because it is a tool used by spammers. In that case the message will be sent from the address you used to login to their server, and the reply-to address will be the alias.
You will need to discus options with your email provider if they do not allow it.
It works with Thunderbird for the same Gmail account