The SMTPUTF8 extension allows servers to send messages that include unencoded UTF8 characters in the MIME headers. This extension is necessary to support Email Address Internationalization (EAI).
When SmarterMail sends a message, it parses the remote server's EHLO response to determine if it supports SMTPUTF8. Based on that, SmarterMail builds the outbound message, applies DKIM signing, and sends the message.
Due to security features like DKIM signing, the RFCs state that a message cannot be downgraded except by the originating server. This can create a situation where SmarterMail sends the message to a gateway that supports SMTPUTF8. The gateway then tries to send the message to a server that does not support SMTPUTF8. Because the gateway is not the originating server, it cannot downgrade the message and must generate a bounce message which it sends to the original server.
We are discussing possible changes that can be made in SmarterMail to minimize the occurrence of bounces related to SMTPUTF8 support, such as a way to disable use of SMTPUTF8 when sending to a gateway.
Andrew Barker
Software Developer
SmarterTools Inc.
www.smartertools.com