When we scan incoming messages for spam and viruses we sometimes reject messages and they get sent back to the sender with a non-delivery report. The error messages are explained in detail here.
Mail from external systems may be rejected by us with a number of different error messages which will be fully explained.
If your mail is being rejected, you should be able to mail firstname.lastname@example.org for help. Mail to that address is not subject to most of the checks we apply to other addresses, so you should be able to get through.
Often we can whitelist you so you can get your mail through to us. This is at best an emergency fix. It only affects mail to us, whitelist entries are liable to be cleaned up after a while, and other sites will block you for the same reasons we have. Fixing the problem is better than whitelisting. See the specific error messages for advice.
Where we make use of external resources such as blacklists, the responsibility for using them is ours. All complaints should be addressed to email@example.com.
Specific Error Messages
Here are details of the meaning of specific error messages and some hints as to what you can do to fix the problems which cause them.