As i understand problem is not related to virtualmin, but i am sure most (if not all) of you had to configure mail, so you may know where is problem.
When i am checking for problems on mxtoolbox, i get one warning:
Result: Reverse DNS is not a valid Hostname
And here is what they say when i press More Info:
More Information About Smtp Valid Hostname
Your Reverse DNS Record (PTR) is not a valid host name. According to email sending best practices, a PTR Record should be a valid host name. If the PTR Record is not a valid hostname, there is a likelihood that you will experience email delivery issues with anti-spam services.
Any ideas how to fix this warning?