Symptoms
A customer created mailbox in the POA Customer Control Panel with non-ASCII characters (e.g. Japanese characters) in a mailbox owner name and configured autoresponder for it.
When sending email to such mailbox via the Thunderbird and Becky! mail clients autoresponder goes into loop, and mail is not being sent.
To stop the loop auto responder has to be disabled and the following NDR (Non-Delivery Report) is returned to the destination mailbox:
Hi. This is the qmail-send program at qsmtp02.provider.com.
I am afraid I was not able to deliver your message to the following addresses.
This is a permanent error; I have given up. Sorry it did not work out.
The problem affects email messages sent via the Thunderbird and Becky! clients.
Cause
This is a bug in the mail client software (Thunderbird and Becky!), it puts actual email address in <> on a separate line when sending message, e.g.:
From: =?UTF-8?B?IumvliDpr5Y8dGVzdHRlc3RAbWFpbGswMS53aW4tdHJpYWwuY29tPiI=?=
<example@example.com>
Affected mail clients violate the RFC2822 "Internet Message Format" which states:
Header fields are lines composed of a field name, followed by a colon (":"), followed by a field body, and terminated by CRLF.
Autoresponder cannot properly handle such cases.
Resolution
Contact the vendor of the email client software to resolve the issue or use mail client which works according to RFC standards.