Unsupported quoted string/literal in header-fld-name

[emersion]

emersion/go-imap#589

Offending implementation: mailo.com

[duesee]

TL;DR: A server only allows an atom in a place where an astring is allowed (and possibly required).

I feel that prefering "atom" over "quoted" over "literal" makes sense most of the time and relaxes this issue already. If a value just can't be encoded as an atom (e.g., due to the use of whitespace), there is nothing we can do without sending invalid messages.