Sometimes I get this when I email my best mate using my favourite service:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of
its recipients. This is a permanent error. The following address
failed:

"xxxxxxxx@ihug.co.nz":
SMTP error from remote server in greeting:
host: zuul.ihug.co.nz:
pub.filter4.content.vf.net.nz
Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means.

It doesn't happen all the time, just often enough to be really irritating. My email provider is GMX and it may well (not sure) have a bad reputation as a spam service, but I don't get this anywhere else and I don't spam and never have. Also, I can find no indication that my email address has been hijacked. I have a feeling this may be a case of an over-zealous filter. Is there anything I can do to stop this from happening? I do have gmail as backup but would prefer not to be blocked.