<kzlyExy/3YBZVUSNURxDqMLjYwWYAVGpn6yogCjhITgfirstname.lastname@example.org> you write:
I note that there is no explicit requirement to treat incoming a-labels
as equivalent with u-labels anywhere in the EAI RFCs. Which means that
you can convert labels before sending, but you cannot rely on the
receiver to interpret the result as desired.
I don't think that 5321 requires that bob(_at_)example(_dot_)com and
be treated the same, either.
Beyond some point, you can't force people to be reasonable.
In the particular case I mentioned, my MTA is Courier which treats
A-labels and U-labels equivalently in mail addresses, but not in
identifiers for authentication. It was easy enough to add both versions
to the identifer table, but I think I'm going to send a bug report to
ietf-smtp mailing list