Some email clients consider our emails as a phishing (in Office 365). The only thing that was missing based on the https://mxtoolbox.com/SuperTool.aspx?action=mx was DMARC entry missing.
I've added TXT DNS record like this (TTL 1hr):
v=DMARC1;p=none;pct=100;rua=mailto:[email protected]
Can it resolve the issue? Is there any specific action required to make it work with Mailgun / GoDaddy?
Thanks in advance for any help/info.
Since you have p=none, it won't resolve the issue, you need to inform the mail server to reject the email. Basically, you're just in monitoring mode with p=none.
What that being said, it sounds like you're saying Office 365 is regarding your emails as a phishing attempt. I think you might be hitting their spam filter based on certain keywords. Send an email to yourself and look at the PCL in the email headers in the outlook account and see what it says.
If you don't know how to see view the headers and just want to make it easy, you can use an inbox tester that will show you the headers.
You can also open a ticket with Microsoft and ask them why the PCL is high on your emails. They might be able to assist.
Link to: Microsoft Sender Support
Another good thing to do is to go through this Email Checker Guide, it touches on every aspect of sending email. You might have a mis-configuration someplace that's causing the issue.