"Error sending email: Could not instantiate mail function."

I am running a site with Joomla 5 and CB, both latest version. I tried to make a small (30+) mass e-mailing.
However I received the warning "Error sending email: Could not instantiate mail function." for every user.
What could be the reason for that? How can I fix it? (the e-mail function as such seems to be working as expected).

Sounds like your email settings in System > Global Configuration > Server > Mail are not configured properly for your server.

(the e-mail function as such seems to be working as expected)

So other emails are being sent? Like registration emails? That seams highly unlikely as that's an error happening at the PHP mailer level of things.

Sounds like your email settings in System > Global Configuration > Server > Mail are not configured properly for your server.
               "(the e-mail function as such seems to be working as expected)"
So other emails are being sent? Like registration emails? That seams highly unlikely as that's an error happening at the PHP mailer level of things.

Well, that is not the answer. The vital e-mail settings are, "Allow e-mail" - YES, Diaallow mass e-mail - NO, and Use PHP-mail". There is not much that unexpectedly can go wrong there.
I also (somewhat later) sent a "Test e-mail"" that arrived as expected and stated that my settings were OK.
So, my conclusion is that; If this is not a problem with CB, and not a problem with Joomla, then it must be a temporary problem elsewhere (with the host? Which they have not confirmed.)
What are CBs criteria for issuing this warning?

 

Test emails send to the domain that sent them so they generally always pass. Within CB > User Management try using the Mass Mailer against a test account that doesn't have the same domain as your site. It will likely fail if mailer settings aren't correct. CB isn't responsible for email sending anymore as we just use Joomla API for this for quite some time now. CB isn't specifically issuing a warning as PHP itself is erroring. You'll need to contact your host for the recommended method, but you may need to setup the SMTP mailer settings in Joomla.