Well perhaps I'm missing something among the messages that were sent, and I admit I dropped that additional advice without really looking at all the messages (since those are what tend to be causes so many of these kinds of errors), but going back a bit now, is it not the same issue where Klaus cannot get his e mail over to Dorothy? I can't see where Barbara - and the list for that matter - figures prominently in this. Unless I'm missing something obvious, or something has transpired offline I'm out of the loop on (?).
If the issue is indeed the recent e-mail failure between Klaus and Dorothy, one of their ISP's mail servers is most likely having a problem "handshaking" with the other side. One may be flagging the other as either being a source of spam, or a channel through which it is delivered. Some e mail server administrators choose to run open relays or proxies; others will refuse anything coming from open ones since spammers often send their junk through these kinds of channels. This is because open relays act as amplifiers and anonymizers for spammed messages [an open relay is simply a mail server that will accept and deliver mail from point A to point B without question].
Many ISPs have begun implementing stricter filter systems on their servers over the last few years, with far greater restrictions in place than in earlier times. This is an ongoing practice, so things can change at any time with them (they all act independently and strictly with their own security interests in mind). So it often comes down to how any given ISP sets up their security settings and filtering baselines, and that is subject to change at any time.
I'd suggest both parties contact their respective ISPs and point them to the rejected e mails so that those folks can examine the header information. This should determine where and what the hang-up is between the two servers. Legitimate mail getting erroneously blocked is nothing new, especially with all the junk mail floating about nowadays. No ISP firm (or server) is obliged to accept mail from anywhere, but if they do reject it, they should at least give sensible and valid reasons. An American based ISP might choose to block perfectly legitimate e mails sent from say a British or German ISP, or it might be just the opposite! Sometimes automated systems create blacklists that are at the root of this trouble. Other times the problem comes from incorrectly issued - and thus unrecognized - dynamic (residential) IP address ranges which will be rejected out of hand by many servers, since those kinds are not allowed direct access. An ISP might also choose to block specific domains. For any of these reasons, you have to ask the ISP to do something about it, as in unblock the block!
Keep in mind, bounced e mails can also come from certain viruses or worms infecting one's system locally, transmission timeouts or DNS caching issues, a malfunctioning or misconfigured (or simply cheap and unreliable) mail server at the host site, or the use of antiquated (non-updated) spammer blacklists. Any of these can cause permanent SMPT error messages, aka "Fatal Errors" (temporary ones are where the server indicates it will retry a number of times to push through a hung message, with no intervention on your part required). However as soon as you see "Permanent Error" in the rejection return, there's little point in attempting to resend the mail, as it is unlikely to change.
Beyond that, all Internet Providers have ISP-specific space and usage limits. Also the sending mail server may require you to authenticate (confirm your identity) to it. Sometimes you need to check with your e mail provider to see that you are properly set up via your e-mail client. One of the ISPs in question may also simply have misconfigured one of their SMTP->SMS gateways, or intentionally locked them down because of excessive SPAM or DDOS attacks. On and on it goes.
As for the clarity of SMPT return error messages, trust me they are not always as precise or clear as you might think (no more than Windows generates clear error codes at all times). The error message one reads on one end may not be spelled out the same on the other. This is generally determined by how a server is set up to begin with. Only the header information on the rejected messages and tracer pings will render a clearer picture. And that is an ISP support dept's responsibility.
Both parties should contact their Internet Service Provider before assuming the fault lies on the opposite side. Whoever has the actual rejected messages (with the requisite header info), start there.
Hope this helps gang. File for future reference!
Jb
PS. Gale shake me good if I'm overlooking something obvious (wouldn't be the first time I needed it).