When I get mail regarding update - where is the site url generated?
I ask because some sites I manage get the correct root (+ /admin/reports/updates) https://sitename but others get inserted a www. in front of sitename - and, worst, some get mail. inserted.
I have been unable to figure out where this come from.
Not a catastrophe but additional time is needed which I would rather 'escape'.
I suspect what is happening is that your .htaccess file is allowing people to go to the site either on sitename or www.sitename
What you get might depend on which url a visitor is using that triggers the update check cron job.
In the Backdrop .htaccess file there is a bit you can uncomment to direct people to one or the other. I recommend doing that.
If you go to mail.sitename in the browser does that also go to your site? If so, you may need to change somewhere and/or add another rule into .htaccess to cater for it.
You can also add $base_url in your settings.php. See
https://docs.backdropcms.org/documentation/protecting-against-http-host-...