The same thing happened to me. I opened a brand-new mail.com account for the first time 2 weeks ago. Sent one email as a test. 2 Days Later I received a message from mail.com stating my account had been blocked for irregular activity. Two days later another message from mail.com stating that their security team decided that my email was to remain blocked and there was nothing more they could tell me about it.
I've used email since it was invented different companies over the years. Never have I had anything like this happen. I've been Googling this issue and just want to State their many people with this problem. The sad part is mail.com gives no option for correcting the situation. No communication whatsoever to find out why they choose to do what they do. S2 my account I emailed myself a test email to another account. Absolutely no reason for them to do what they did.
My thoughts to those reading this post, stay away from mail.com. they're absolutely absurd in there security practices. Gmail seems the way to go. Only reason I tried mail.com was because the name I wanted to use was already taken up at the other email services. Mail.com had my requested name available. No way to do business mail.com. Shame on you no. If I could I give you three thumbs down.