The following servers have closed themselves to mail from signalbox.org since this forum was created, rejecting all communcation with a 441X error. This indicates that they cut off communication as soon as they identify the source.
orangehome.co.uk (this provider simply blocks our mails without response)
This is because they identify the repeated email notifications generated by this forum as junk mail. They do not let it reach you, or advise you so you can decide for yourself whether it is junk and act accordingly.
Additionally, BT Internet/Yahoo have introduced a deliberate delaying system for mail from "bulk mail" sources (see http://help.yahoo.com/help/us/mail/defer/defer-06.html) in order to see if the messages are re-sent (most junk mail is not) and this results in my receiving bounce messages too.
If you are using any of these providers, please turn off all email notifications as for every one you are sent, as I personally get a bounce message each time. (You can do this through your profile). If necessary, to stop me being bombed by your bounce messages, I will disable accounts - in which case you will have to contact me for it to be re-enabled when your email is again working.
Contact your provider and tell them to stop interfering with your email. Let me know when they have fixed it so that I can take them off the above list. If you don't bother to let me know, the domain will remain listed above even if the problem is solved.
Other straightforward mail bounces will also result in accounts being disabled - if your email address is not valid (ie the mailbox has been shut down temporarily or permanently) you are not entitled to be a member of this forum - you must have a valid email address.