Sending an email from a sender address (email@example.com) to a specific reveiver address (firstname.lastname@example.org) I get the following reply:
Is there anything I can do to prevent my mail to end up in the receiver's spam?
Not on your end. That's the idea behind a spam filter. They need to unblock you on their end.
It's entirely possible your mail server is blacklisted.
I have a Sendgrid.net account. I use it to send out newsletters. A low-volume account is free. I believe you can set up a mail client to use it as an SMTP server for your outbound mail.