ok, i executed your code snippets in the horde php console and it 
worked. i checked the Core/ActiveSync/Driver.php for a call something 
linke "->toArray(array('charset' => 'UTF-8'))", but there was not 
something like that. i checked the file in the git-repo and noticed a 
lot of changes.
i am not sure but i believe your changes 
(f309400f86940b7bc74ca48472420464d08bbf7b) did it.
i upgraded horde and now it works.
Thanks for your patience. issue can be closed.
Jens

Also, regardless of how the client sends the email, 
Horde_Mime_Headers->toArray() should properly encode it anyway. What 
does the following output for you?

$hdr = new Horde_Mime_Headers();
$hdr->addHeader('To', 'Michael Rübinsky <mrubinsk@horde.org>');
var_dump($hdr->toArray(array('charset' => 'UTF-8')));

For me, it correctly outputs the following for the "To" value:

"Michael =?utf-8?b?UsO8Ymluc2t5?= <mrubinsk@horde.org>"

If this doesn't work for you, then either your git checkout is not up 
to date, or your php is broken.

That's because that is not a valid RFC822 email address.  My guess is 
that your client is sending it incorrectly. What AS client are you 
using?

For reference, sending an email to Michael Rübinsky 
<mike@theupstairsroom.com> results in the following at the same debug 
point you used:

string(59) "Michael =?utf-8?b?UsO8Ymluc2t5?= <mike@theupstairsroom.com>"

Hi,
i  added some debug output to Transport.php (line 183) to see what the 
content of the variable $recipients is, which is passed to the 
parseAddressList function.
$recipients = "Günter <jens@sg-dev.de>";
So i created a litte demo script called test.php:

<?php
include 'Rfc822.php';
include 'Exception2.php';
include 'Rfc822/Object.php';
include 'Rfc822/Address.php';
include 'Rfc822/List.php';

$rfc822 = new Horde_Mail_Rfc822();
$recipients = "Günter <jens@sg-dev.de>";
$addresses = $rfc822->parseAddressList($recipients, array('validate' 
=> true));

?>

The same exception like in my log file is thrown. calling with 
$recipients = "Gunter <jens@sg-dev.de>"; works.

Note: in my test script i included Exception2.php which i copied from 
Exception.php and changed it to:
class Horde_Mail_Exception extends Exception
{
}

So i do not have any Horde dependencies to get my test script executable.

Unable to reproduce using all kinds of non-ascii characters.

What is the exact email address that is causing validation to fail?

if an address contains german umlauts sending emails via ActiveSync 
fails. This happens for the "From"-address and for the 
recipient-addresses. Sending without umlauts works fine.

The active sync log contains the following enries:

2013-01-02T14:08:59+00:00 INFO: Request being handled for device: 
androidc1264404651 Supporting protocol version: 12.1
2013-01-02T14:08:59+00:00 INFO: [androidc1264404651] Handling SendMail 
command.
2013-01-02T14:08:59+00:00 ERR: Error when parsing address list.
2013-01-02T14:08:59+00:00 ERR: exception 'Horde_Mail_Exception' with 
message 'Error when parsing address list.' in 
/var/www/horde5/pear/php/Horde/Mail/Rfc822.php:261
Stack trace:
#0 /var/www/horde5/pear/php/Horde/Mail/Rfc822.php(164): 
Horde_Mail_Rfc822->_parseAddressList()
#1 /var/www/horde5/pear/php/Horde/Mail/Transport.php(186): 
Horde_Mail_Rfc822->parseAddressList('Jens G??pfert <...', Array)
#2 /var/www/horde5/pear/php/Horde/Mail/Transport/Smtp.php(222): 
Horde_Mail_Transport->parseRecipients('Jens G??pfert <...')
#3 /var/www/horde5/pear/php/Horde/Core/ActiveSync/Driver.php(1085): 
Horde_Mail_Transport_Smtp->send('Jens G??pfert <...', Array, Resource 
id #262)
#4 /var/www/horde5/pear/php/Horde/ActiveSync/Request/SendMail.php(54): 
Horde_Core_ActiveSync_Driver->sendMail(Resource id #242, false, false, 
false, true)
#5 /var/www/horde5/pear/php/Horde/ActiveSync/Request/Base.php(213): 
Horde_ActiveSync_Request_SendMail->_handle()
#6 /var/www/horde5/pear/php/Horde/ActiveSync.php(639): 
Horde_ActiveSync_Request_Base->handle()
#7 /var/www/horde5/pear/php/Horde/Rpc/ActiveSync.php(150): 
Horde_ActiveSync->handleRequest('SendMail', 'androidc1264404...')
#8 /var/www/horde5/rpc.php(155): Horde_Rpc_ActiveSync->getResponse(NULL)
#9 {main}
2013-01-02T14:08:59+00:00 ERR: exception 'Horde_Mail_Exception' with 
message 'Error when parsing address list.' in 
/var/www/horde5/pear/php/Horde/Mail/Rfc822.php:261
Stack trace:
#0 /var/www/horde5/pear/php/Horde/Mail/Rfc822.php(164): 
Horde_Mail_Rfc822->_parseAddressList()
#1 /var/www/horde5/pear/php/Horde/Mail/Transport.php(186): 
Horde_Mail_Rfc822->parseAddressList('Jens G??pfert <...', Array)
#2 /var/www/horde5/pear/php/Horde/Mail/Transport/Smtp.php(222): 
Horde_Mail_Transport->parseRecipients('Jens G??pfert <...')
#3 /var/www/horde5/pear/php/Horde/Core/ActiveSync/Driver.php(1085): 
Horde_Mail_Transport_Smtp->send('Jens G??pfert <...', Array, Resource 
id #262)
#4 /var/www/horde5/pear/php/Horde/ActiveSync/Request/SendMail.php(54): 
Horde_Core_ActiveSync_Driver->sendMail(Resource id #242, false, false, 
false, true)
#5 /var/www/horde5/pear/php/Horde/ActiveSync/Request/Base.php(213): 
Horde_ActiveSync_Request_SendMail->_handle()
#6 /var/www/horde5/pear/php/Horde/ActiveSync.php(639): 
Horde_ActiveSync_Request_Base->handle()
#7 /var/www/horde5/pear/php/Horde/Rpc/ActiveSync.php(150): 
Horde_ActiveSync->handleRequest('SendMail', 'androidc1264404...')
#8 /var/www/horde5/rpc.php(155): Horde_Rpc_ActiveSync->getResponse(NULL)
#9 {main}
2013-01-02T15:09:04+01:00 DEBUG: [27658] Initializing state for 
collection: INBOX, synckey: {50bc7406-5c54-4dfb-a993-14613e4b951e}493
2013-01-02T15:09:04+01:00 DEBUG: [27658] Loading state for synckey 
{50bc7406-5c54-4dfb-a993-14613e4b951e}493