Email Messages getting blocked at Microsoft Hosted Mailboxes

There might be several reasons for this kind of issue. We moved step by step to resolve this issue.

(1) Added Reverse DNS :

First of all, we have mapped the IP address with the mail.domain.com. This is called rDNS or PTR record. It is essential for email delivery, so the recipient server can validate your mail domain that it is coming from the valid IP address.

(2) Asked to use VPS IP for Authentication :

It is always recommended to send emails using the authentication of your server where you have hosted your mail domain. Thus, we advised the client to modify the mail script and use the IP address or mail server hostname for the proper authentication. Also, added email account and password of one of the mail domain users in the script.

(3) Modified email sending code :

Finally, we discovered that emails with the plain text were delivered without any problem but not the HTML emails. We looked at the differences in the code and changed the HTML version SMTP section to match the plain text version. This section of code is related to the authentication of the message. When we changed it to match the plain text version, the emails were delivered to Microsoft servers successfully.