Email rejection?

Getting this message for some of my site members, is this just something that the receiver needs to resolve?

Connected to but sender was rejected.
Remote host said: 451 4.1.8 Possibly forged hostname for

My guess is the sender is using his own ISP’s SMTP server to send mail (many block port 25 to all servers but theirs as an anti-spam measure), and the recieving server is seeing in the sender line but the mail is coming from and the recieving mail server sees this and it doesn’t jive and thinks it spam.

Take a look at this

Hope this helps, though not sure what you can do about it.