Hotmail (Microsoft) is notorious for blocking email servers for suspected spamming. Your hosting company may be able to check whether their email server is thus listed, and apply for reinstatement.

Ok thanks !

It also depends on the correlation email/server.

For instance, if your email is sent by your server but it uses a senders email of something@hotmail.com or something@gmail.com than the receiver might block it because the service sending the email is not the owner of the email domain. So it looks fishy.

Sorry for the difficult to understand explanation, I have trouble finding my words today...

Thanks, I finally just changed my email in the user database and got it sorted!