EMail service lost when moving to CloudFlare, unknown MX record


I wondered if anybody could help me understand what went wrong in the following:

TEST message, that forum tool is bugged…


In fact trying to post the following :

I moved a domain “mydomainDOTfr” to Cloudflare by updating the DNS NS at the registrar. For other domains tested before, I realized the “Copy existing zone records” feature missed crucial records sometimes; strange but seen before moving. Not this time though: nothing missed and the domain had very simple records: A zone, Cname for www and 1 MX entry “mailDOTmydomainDOTFR” which had an A record for mail. my domainFR and worked fine.

The day after starting to use Cloudflare and getting the mail confirmation that everything was fine, the users started complaining that people told them their email bounced. On studying bounced headers, the clients providers said they could not deliver mail to “_dc-mxDOTabcd132kdiue457feDOTmydomainDOTfr” (not the real chain but close). That entry for MX never existed, it was not in the previous DNS service, and was not created nor of course visible in the Cloudflare panel.

I then proceeded to creating an MX and A record for that strange value but it seemed to never exist/propagate. Users kept losing emails at the time. I deleted the existing unique MX record to recreate it but it took hours to be visible again. I finally resorted to leaving CF DNS to restore the domain on my previous service so emails would come back.

All in all a big stress and failure; not understood either. Does anybody have some light to shed on this issue ? Major point to solve before I decide to switch to paid plans and use CF for critical domains.

Edit: added space in example domains as the forum forbids links.
Edit: now that tool stupidly says “new users can only put 4 links in a post” whereas mine has none… I understand spam must be fought but that should not prevent us from communicating…
Edit: Had to post that message in chunks to not be blocked…

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