To use the alias, as you call it, you need an identity with an outgoing server (SMTP) configured for that identity. Have you added one for each identity?

Thank you for getting back to me, Matt. I did try to find my question among the lists of questions already answered but failed. Your reply made me question my understanding of the terminology so I looked for some background information. And immediately found my exact question. The post was old but still valid and the instructions included how to add identities. Why there are different terms for in coming and out going is beyond me but I'm sure it makes sense to you. Thank you for nudging me in the right direction. gary

There are primarily different terms because the internet is a global community and US marketers (Microsoft and US based ISP's particularly tend to use US terminology to appeal to their US customer base. Just as US television remakes international shows so cater to their customers biases and morals.

The alias is entirely an invention of US marketers. It is another email address in every instance I have ever seen. It actually has no basis in the standards that control email. Purely a marketing term.

Similarly the identity is also something that is not "standards" defined. But, at least in my opinion, it does not have the overtones of illegality that an alias conjers in my mind. But it is also a very old term, dating from the days of Netscape in the 1990s where it was coined.

The identity has many functions, primarily for those that aggregate email to a number of mail addresses into a single account and then download that using Thunderbird. It allows each of the aggregated mails to be replied to using appropriate names and titles as well as from address without user intervention. Hence each identity having an outgoing server associated with it (including the primary identity.)

You are trying to use a manually selected sender, and to do so an identity is required. But in many cases if the identity is correctly set up, Thunderbird will select the correct identity when you click reply.

One final point. Despite everyone, or nearly everyone, having a desire to get and send mail. The two processes are actually not related either technologically or practically. Getting mail is akin to getting water into your house, while sending it is akin to the sewer in that analogy. They work together (generally with an air gap between tap and plug) but each work entirely independently from the other and one is not required for the other to function. So the makers of mail clients have been dealing with this disparity of similar actions for years and trying to dumb it down. Microsoft chose to pretend that sending and receiving all happen as a single thing. ie you "set up an account". Netscape acknowledged that they generally happened together but not always using identities and offering the option of multiple outgoing server setup per incoming account if the user chose to do that. Neither approach was wrong, just each took a slightly different approach to the same problem. One empowered the user at the risk of confusing, the other made it as simple as possible but restricted choice.

Thank you for the for the lesson, Matt. Interesting how terminology evolves over time. I do remember Netscape and even Mosaic. I understand the need to use precise nomenclature for specification and for answering questions. But as a user; under the hood it's still magic. later, gary