This is not intended for display to end-users.  It's intended as a 
human-readable message to the calling client, since SMTP clients are 
not necessarily MUAs.  Calling clients can be things like relaying 
MTAs, for example.

SMTP error messages are not designed to be automatically displayed to 
the user.  This is not my opinion; this is the way it is designed.   
See, e.g., PHP Mailer which does the same thing we do: display 
translated error messages based on the error response codes: 
https://github.com/PHPMailer/PHPMailer/tree/master/language

Postfix, for example, is allowing SYSTEM ADMINISTRATOR to set custom 
error messages for showing to USERS. How errors set by ADMIN are 
supposed to be shown only to ADMINS (i'm using your logic now) ?  I 
believe the most SMTP servers have such a feature.

Here is a list of SMTP extended status codes (not all of them are errors):

http://www.iana.org/assignments/smtp-enhanced-status-codes/smtp-enhanced-status-codes.xhtml

The vast majority of these messages have absolutely no business being 
shown to an end-user.

Are they useful for an admin?  Yes (possibly).  That's why they are 
logged to the Horde log.

Completely false.

If you are a non-English user, and you are a presented an error 
message in English, I fail to see how this makes things clearer to a 
user.  And that doesn't include those users who can't understand Roman 
text at all.

I was just in Beijing last month meeting with the people that 
implemented the controlling docs on e-mail address 
internationalization.  One of the things they didn't do when drafting 
is to allow SMTP error message to be internationalized (unlike POP3 
and IMAP) because those error messages are NEVER supposed to be 
displayed to a user (in other words: SMTP has no language support).

So this is not just some crazy idea of mine - its the underlying 
design of SMTP.

Michael, i really like the way you ignore your users ;)

so what about to make it configurable?

We are using this to print a messages to users when we disable their 
accounts (e.g. Your account was disabled due to spam). Anyway, it is 
always more usefull if a user calling to tech. support is able to tell 
an exact error message (no matter in which language it is). All e-mail 
clients are able to print exact error message from SMTP server and 
they are doing it because it's usefull.

And this is the expected behavior.

Not very useful to print out SMTP errors to the user if there is no 
guarantee that 1) they are decipherable to a non-techincal person or 
2) in a language that the user understands.

The actual SMTP error text is output to the Horde logs.

IMP stopped showing SMTP error messages returned by SMTP server - it 
just prints out generic message like 'Server is not accepting SMTP 
connections'.