You just quoted two methods, both using HTML and then asserted you need to write in HTML.

The first version, used by Thunderbird allows the rendering engine to mke the font one "size" smaller than "normal" for the device.

Your HTML will try and force your perception of beauty and readability on those using 46 inch screens and those using tiny phones. It is much better to allow the font to be stretched and shrunk based on the display. The object is to get folks to read your mail. They will not thank you if it is clownishly large or microscopic when they get it. This is one reason Thunderbird has an option to allow the user to ignore the font settings you specify,

As for how you do it. Install one of the add-ons that allow access to source and and create your own template that you use religiously. Also make sure you never ever copy anything from a word processing package or a web page as anything but plain text, as that will upset your carefully crafted HTML fonts.

Thank you Matt for your response, unfortunately, it's not helpful. I understand your point, but the fact is, the font tag has been deprecated for a reason: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/font

I'm trying to introduce Thunderbird in my current company, and the most frequent complaint I get is the lack of consistency in the appearance when replying to messages written in Outlook. We need to use a corporate style so messages look the same, regardless whether they are composed with Outlook, or Thunderbird. The fact is, people have gotten used to the control over the appearance which Outlook has and, I'm sorry to say, Thunderbird lacks.

... It is much better to allow the font to be stretched and shrunk based on the display. The object is to get folks to read your mail. They will not thank you if it is clownishly large or microscopic when they get it. This is one reason Thunderbird has an option to allow the user to ignore the font settings you specify,

IMHO this is just your opinion, totally dependent on the use case. Ideally, we need to have a common set of styles (css) which can easily be used when composing messages. Our messages are not read in anything other than a workstation running Windows.

As for how you do it. Install one of the add-ons that allow access to source and and create your own template that you use religiously. Also make sure you never ever copy anything from a word processing package or a web page as anything but plain text, as that will upset your carefully crafted HTML fonts.

Could you recommend one of these add-ons?

TIA,

Pedro.

I use https://addons.thunderbird.net/en-US/thunderbird/addon/thunderhtmledit/

but mail merge also has one https://addons.thunderbird.net/en-US/thunderbird/addon/mail-merge/

Thanks again Matt, unfortunately these add-ons don't solve my initial question of controlling how the default template looks.