No mail is sent (neither via php mail() nor via swiftmail (yii2)). Why?

Question

PHP

No mail is sent (neither via php mail() nor via swiftmail (yii2)). Why?
Not to any email.
Where in general in this case to dig?
PHP 7.0
php error log is empty...

Answer 1

Maybe the host blocked this feature due to spam complaints.

Answer 2

I found something in the logs:
Googling led me here:
https://wiki.debian.org/PkgExim4UserFAQ#I_get_the_...
from where it became clear that Exim is configured to work locally by default.
True, it is not clear what Exim has to do with it if php works through sendmail.