2021:
O365 szervereken keresztüli küldés rDNS checken fennakadnak.
https://docs.microsoft.com/en-US/exchange/troubleshoot/antispam-and-pro…
forumból:
Regarding your situation, I want to tell you that setting up a PTR record is not supported by Office 365. Thus, it's not a bug and the truth is that your recipient domain has rejected your message. Office 365 is a shared environment that provides email service and the IPs are variable rather than fixed when sending emails via Office 365 SMTP server, it will return errors when the recipient email sever is using reverse DNS check.
https://answers.microsoft.com/en-us/msoffice/forum/all/misconfigured-pt…
the error messages says that the recipient (your email address) side checks the PTR record of sender (Office 365 email address) and it failed, however, Office 365 doesn’t support the PTR record management, so Office 365 account can’t send emails to your side. Note: For this issue, please help add Office 365 email address into white list/allowed list from the your side (Hostgator) and check if the issue persists. You can also contact Hostgator to add the Office 365 domain to allowable list to see whether this issue persist.