Well I suspect they didn't configure the reverse DNS correctly - it can be tricky. Is it just email hosting or also websites?
You could consider just switching your email hosting to another provider but keep your websites were they are so you don't have to move that. (assuming your doing both)
I have email hosted with hostinger titan for one of my domains - I set it up a long time ago and haven't bothered to switch to my local server for email.
But they've been rock solid on email.