I have never experienced this type of issue before, so I don't know where to begin searching to be able to troubleshoot or fix the problem. Also, I hope this is the correct SE website for this type of question.
I launched a website last month for a friend's business and helped him set up his email, etc. The site is hosted on my turnkeyinternet.net reseller account. It is the Professional I account.
A few days ago he needed to add SSL and, obviously, a dedicated IP. The reseller account comes with 2 free dedicated IP addresses and a free OneClick SSL.
I gave him one of my dedicated IPs and the OneClick SSL. TurnKey set up both of these for me. The IP two days ago and the SSL yesterday. Today, my friend called me and told me that one of his clients was getting a bounce-back email when trying to send him an email at his domain name. His client said it first happened two days ago (the same day TurnKey set up the dedicated IP).
This was from a @hotmail.com account. I began testing and found that this issue was with all Miscrsoft affiliated addresses: Hotmail, Yahoo, Outlook, Live (and I assume any professional email addresses hosted by these services or that use these as an email client).
I confirmed with my friend AND his client that prior to the changes made by TurnKey, they were both sending and receiving each other's emails.
I contacted TurnKey and their response was: "Nothing has changed on our end as far as the mail server/DNS. It seems these providers are simply blocking email sent to the IP in question. Yahoo/hotmail are blocking the emails so it's up to them to whitelist or allow them to go through."
His website does not have a newsletter and he hasn't sent mass mail.
It is obvious to me that whatever changes TurnKey made caused this issue. They will not help me and I don't know where to begin to troubleshoot.
I received an email from TurnKey this morning stating, again - "This has nothing to do with any IP or SSL's being recently changed/updated/installed or otherwise."
However, my friend called me and said he tested everything again this morning and everything is working correctly with Microsoft email addresses.
This makes me wonder if TurnKey discovered their mistake, fixed it, then lied about making the mistake.