identification strings on address literals

[novedevo]

2821 allowed this, but it was never properly used and often caused errors. It's "probably wise" (note the lack of SHOULD) for us to be prepared for such strings, but we should disregard them if they occur.

[gennyble]

The only place I can find a mention of this is in section 4.4 about trace information, maybe?

Could you elaborate some more or provide a section number?

[novedevo]

4.1.1.1 claims:

RFC 2821, and some earlier informal practices, encouraged following
the literal by information that would help to identify the client
system.  That convention was not widely supported, and many SMTP
servers considered it an error.  In the interest of interoperability,
it is probably wise for servers to be prepared for this string to
occur, but SMTP clients SHOULD NOT send it.