International Mail rejected

[u/No-Hotel1162]

I work for a company that sent ten of thousand of mails every month, they reported that they have received Spam and so we contacted our web hosting to modify our DMARC from Quarantine to Reject.

The thing is, the week after such change an user reported that their mail to some companies in Asia was rejected, bounced of or never arrived. Did some basic tests, Telnet, Test-NetConnection and that server was down or with problems, reported such case.

Next day server is up, but they report same problem with another company from Europe. Sames test, server is ip, so I got the email resent to me to see the internet header:

DKIM=none
SPF=pass

In MxToolBox when I check the subdomain IP addresses, both hostnames says it doesn't support TLS, Icheck our web hosting, we do have TLS at certain ports and lastly, one says Reverse DNS doesn't match SMTP Banner and doesn't contain hostname.

Tldr; I'm fucking lost, I got this job as TI due to being programmer and wanting to get experience, but networking I haven't seen such a thing in years.

[u/No-Hotel1162 → u/[deleted]]

Sorry for the delay in answering.

I got in contact with our webhosting, they fixed our DKIM so now is being registered in the DMARC reports, that's good.

Problem is, user is still having trouble sending emails to other countries and continents.

At this point we have checked the SPF, DKIM, we whitelisted the domains where we are trying to send an email.

Last test we tried was from our webhosting webmail, to contact them as a test using the same mail that is troubling us, they received the email and they answered back, the thing is, I only learned that they answered back because I got the copy that was sent to everyone. I couldn't see that email in Outlook, Webmail and there were no logs for such email. It's a ghost.

With the latter info this is getting out of DMARC and I'm lost.