Thanks for your reply!
In your link I found this:
Make sure your server is not blacklisted
If your server is blacklisted, your e-mail will be considered as a spam e-mail.
You can make sure your server is not blacklisted on this website :
www.mxtoolbox.com/blacklists.aspx
If you're server is blacklisted, you have two possibilities :
* Fix it! (usually it's a problem of reverse DNS)
* Use an external SMTP Server so your e-mails are not sent from your own server any more.
I'm not an expert... Can you please explain how I can use an external SMTP Server so that my mails are not sent from my own server anymore? What parameters should I set?
Thanks