Problems with the header part 'to' on certain servers

Hello,
a customer of us has a problem to send an E-Mail to a certain account hostest by 1&1. The E-Mail is thrown back as the to field is not RFC compliant, exact error message:

Reason

Your e-mails have been rejected by our mail system because the information provided in the e-mail header does not comply with the specifications in RFC 5322 and RFC 2047.

The header field "To" is syntactically not correct.

Solution

Please make sure that the requirement of RFC 5322 chapter 3.4 for the header field "To" is fulfilled.

In case you use Non-ASCII characters like accented letters or symbols, please consider RFC 2047. For these characters you must use MIME encoding.

As soon as the corrections have been made by you, you can deliver emails to our system again.

The customer forwared be the e-mail and I checked the addresses, there are no special characters, which are not allowed, they enter most addresses in the following form:
'm-mustermann@domain.com' m-mustermann@domain.com; 'office@domain2.com' office@domain2.com

I have read the mentioned chapter 3.4 and did not find anything which is not fullfilled.

Thanks,
Florian

Had someone the same problem, and found a solution?

WalterH On which site? It is hard to believe that a such a huge provider like 1&1 has a misconfigured spamfilter. However did you find a solution?

Thanks,
Florian