How do I fix this: mail doesn't work. But on my other sites (MSM) it works just fine

I hope someone can help me out because it’s giving me a headache.

I am using EE version 2.11.5 and have a couple of websites running in a MSM environment. On all sites mail works fine except for this one site. Can’t figure out why.

The email adres (in email config) is a valid one by the way.

• the testmail.php from /utilities works fine.
• the communication email (tools > communicate) works fine also.

But:

• forgotten password reset: not being sent
• forms: not being sent
• new entry update: not being sent

So I know it’s not the server. Must be something in EE. Tried Sendmail and SMTP but all fails.

Thank you in advance.