Tried this?
Configuring a DKIM record
https://help.ovhcloud.com/csm/en-ie-dns-zone-dkim?id=kb_article_view&sysparm_article=KB0058259
I think the error message is in this part:
5.1.9 (DKIM reject DKIMr) Missing, invalid or non-matching DKIM signature_
Please let us know if it resolved your problem.
Also you may want to have a look at my post (link below) that I believe has some useful info and
where I have looked into how to get free OVH e-mail
compliant with the DKIM/MTA-STS, but that is impossible
as OVH offers one e-mail address for free but my e-mails end up in SPAM
folders or rejected because of missing ability to configure DKIM on free e-mail address.
Would German equivalent of this be a car without a dashboard? I can drive it, but would not know
how fast I will be going, what my fuel level is etc. as with the e-mail solution for free e-mail account or e-mail without DKIM I will not know if the recipient seen my e-mail in Spam folder (let's face it, who checks their Spam filter)?
https://community.ovhcloud.com/t/6565