Vps smtp

Hello BHW members,

Couple of days ago I was able to send emails from my server's SMTP, after sending number of emails for testing purposes, The SMTP stopped sending emails to Hotmail, Gmail and Yahoo except AOL, The sender says that the messages were sent, but after checking the inbox of the three providers I couldn't find anything even in the spam box, and after that I discovered that the emails were sent only to Yahoo after 24h or so.

I checked if my server's IP was blacklisted but it was fine.

So what could be the problem?

Thanks.