I am using Exchange Online (custom domain) for my wife's small business email (two mailboxes, 3 email addresses), and she is having trouble sending email to a few different service providers (i.e. Contacting her PAYING clients), the main culprit being RoadRunner. Since RoadRunner (columbus.rr.com specifically) is the largest ISP in our area, this is becoming quite a problem.
I checked with them and they said the issue is that there is no reverse DNS entry for my wife's MX record.
The FAQs and posts I've found around here suggest asking to be added to a whitelist, but RoadRunner's FAQ says they don't manage their own whitelist, they use ReturnPath. So, I went to ReturnPath. All I can find in their FAQs is that they don't offer "Certification" (whitelisting) for shared mail servers.
So now what? My wife is unable to contact her clients via email if they are using the largest ISP in the area, and there seems to be no solution other than ditching the Exchange Online service.
I guess I should add that she does not send out any "bulk" email. At all. She only uses email to respond to inquiries for new business and/or contact existing clients regarding ongoing services.