Ran into problems sending Mail. Response: 550 message to verify they are valid

[Divvy]

I always use my phpBB forum to send mass emails for my users, and always works fine!
But today, I´ve got this error:

Ran into problems sending Mail. Response: 550 message to verify they are valid.

Any suggestion that what could be?

Someone told me that:

There's been a relatively recent update of cpanel. In the past, the server would accept the e-mail and then spit back a 550 message to the sender.
Because of all the spam sent from invalid accounts (I believe), cpanel has now been configured to refuse mail to failed addresses at SMTP level. The bounce message a sender receives is from his sending SMTP server.

Of course, many of the moderators are using their webserver as their SMTP server, so they wouldn't necessarily see this change that easily.

It is true?
How can I fix this problem?

[bking]

Whats in the logs?

[Divvy]

Whats in the logs?

What logs? where can I find them?

[bking]

/var/log/exim_mainlog
/var/log/exim_rejectlog