Welcome!
I think we are missing the reason why OVH sent you in that direction, but I’m guessing it is due to the open resolver implemented by OVH?
cf. Trouble with spamhaus marking any mail as spam - #2 by Maniack_Crudelis
I do not like the solution of commenting out the lines in the postfix configuration. On my own server I made sure that the cloudconfig set by OVH to automatically set the network config and stuff does not alter the /etc/resolv.conf file.
This file should only refer to nameserver 127.0.0.1, pointing to your own local DNSmasq resolver.
Try that, and revert the MX list to the one suggested by YunoHost 