Changes have been made in Git (develop):

commit 0a3d57b3ff73564b79bf35640c3c34bbf9a1732a
Author: Jan Schneider <jan@horde.org>
Date:   Tue Apr 17 15:21:30 2012 +0200

     Revert "[mms] Fix regression in sending PGP encrypted messages 
(Bug #11085)."

     This reverts commit c5540771fc376b2e770d49f8a7a4e0cdfacf65ba.

     Conflicts:

             imp/docs/CHANGES
             imp/lib/Compose.php
             imp/package.xml

  imp/lib/Compose.php |   29 +++++++++++------------------
  1 files changed, 11 insertions(+), 18 deletions(-)

http://git.horde.org/horde-git/-/commit/0a3d57b3ff73564b79bf35640c3c34bbf9a1732a

I tracked the code and can't for the life of me see how such a struct 
would end up in the line.
Are you sure you are using the latest IMP *including* my recent 
commit? Are the other libraries up-to-date?
If yes, you need to track down yourself, where this array is coming from.

It's definately weird. Although others had the exact same problem [1] 
& [2], not all seemed to be able to use my patch. But I hoped that my 
proposal will be properly verified and cleaned up if necessary, as I'm 
not very familiar with the horde framework, nor a php programmer. 
Which is what I thought Michael did.

Anyway, let's properly figure out, what goes wrong here and get a fix 
for it. We should probably reopen that ticket, or create a proper one.

[1] http://comments.gmane.org/gmane.comp.horde.user/27529
[2] http://comments.gmane.org/gmane.comp.horde.imp/29408

Added a
   Horde::debug($to)

statement right in the beginning of _createMimeMessage.

2012-04-18T00:44:01+02:00 DEBUG: Variable information:
array(1) {
   [0]=>
   array(5) {
     ["comment"]=>
     array(0) {
     }
     ["host"]=>
     string(10) "example.com"
     ["mailbox"]=>
     string(2) "foo"
     ["personal"]=>
     NULL
     ["route"]=>
     array(0) {
     }
   }
}

Backtrace:
1. IMP_Ajax_Application->doAction() 
/var/www/vhosts/horde.example.com/www/services/ajax.php:60
2. Horde_Core_Ajax_Application->doAction() 
/var/www/vhosts/horde.example.com/www/imp/lib/Ajax/Application.php:95
3. call_user_func() 
/var/www/vhosts/horde.example.com/pear/php/Horde/Core/Ajax/Application.php:95
4. IMP_Ajax_Application->sendMessage()
5. IMP_Compose->buildAndSendMessage() 
/var/www/vhosts/horde.example.com/www/imp/lib/Ajax/Application.php:1583
6. IMP_Compose->_createMimeMessage() 
/var/www/vhosts/horde.example.com/www/imp/lib/Compose.php:614
7. Horde::debug() 
/var/www/vhosts/horde.example.com/www/imp/lib/Compose.php:1162

If I should provide you with any further information, please let me know.

This doesn't make any sense, webmail *is* the apps installed 
individually. The only difference is that the address book preferences 
in IMP are pre-set. But that doesn't change the data exchanged between 
Turba and IMP.

Track down where the data is coming from.

Horde::debug() documentation: http://wiki.horde.org/Doc/Dev/DebugH4

Also note the remark in #11150 comment 1:
So there seems to be some kind of difference what one install.

Simply reverting Michael's commit (which is an adaption of my proposed 
patch) won't solve this problem. So can we reopen that ticket? PGP 
Encryption is imho in that way still broken, but just at another part.
I don't know how AddressObjects arrive at that part of the codebase, 
but I figured that out, by using var_dump at that location. What I can 
confirm is that my patched solved our problem and works well.

Note my provided log warning about array_flip that fails, how would 
that appear if we would receive an array of strings?
We simply installed horde webmail via pear and had this problem. No 
other modifications were made.

How can we debug that further and provide you with the necessary 
information to fix it properly?

Changes have been made in Git (master):

commit 0a3d57b3ff73564b79bf35640c3c34bbf9a1732a
Author: Jan Schneider <jan@horde.org>
Date:   Tue Apr 17 15:21:30 2012 +0200

     Revert "[mms] Fix regression in sending PGP encrypted messages 
(Bug #11085)."

     This reverts commit c5540771fc376b2e770d49f8a7a4e0cdfacf65ba.

     Conflicts:

             imp/docs/CHANGES
             imp/lib/Compose.php
             imp/package.xml

  imp/lib/Compose.php |   29 +++++++++++------------------
  1 files changed, 11 insertions(+), 18 deletions(-)

http://git.horde.org/horde-git/-/commit/0a3d57b3ff73564b79bf35640c3c34bbf9a1732a

$to *is* an array of address strings. How did you get address objects? 
This change actually breaks encrypting for the senders, see bug #11150.

Changes have been made in Git (develop):

commit c5540771fc376b2e770d49f8a7a4e0cdfacf65ba
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Mar 27 13:00:24 2012 -0600

     [mms] Fix regression in sending PGP encrypted messages (Bug #11085).

  imp/docs/CHANGES    |    1 +
  imp/lib/Compose.php |   29 ++++++++++++++++++-----------
  imp/package.xml     |    2 ++
  3 files changed, 21 insertions(+), 11 deletions(-)

http://git.horde.org/horde-git/-/commit/c5540771fc376b2e770d49f8a7a4e0cdfacf65ba

Changes have been made in Git (master):

commit c5540771fc376b2e770d49f8a7a4e0cdfacf65ba
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Mar 27 13:00:24 2012 -0600

     [mms] Fix regression in sending PGP encrypted messages (Bug #11085).

  imp/docs/CHANGES    |    1 +
  imp/lib/Compose.php |   29 ++++++++++++++++++-----------
  imp/package.xml     |    2 ++
  3 files changed, 21 insertions(+), 11 deletions(-)

http://git.horde.org/horde-git/-/commit/c5540771fc376b2e770d49f8a7a4e0cdfacf65ba

https://github.com/duritong/horde/commit/060d02e78824a420235e23327c5883b0ee28fcf2

All e-mails that should be encrypted got only be encrypted with
the senders public key.
This happened, because $to wasn't an array of addresses but rather
an array of AddressObjects, leading to the following warning, when
sending an encrypted e-mail:

WARN: HORDE [imp] PHP ERROR: array_flip() [<a 
href='function.array-flip'>function.array-flip</a>]: Can only flip 
STRING and INTEGER values! [pid 1982 on line 1352 of 
"/var/www/vhosts/horde.example.com/www/imp/lib/Compose.php"]

However, this also means that all the recipients are omitted and
the e-mail is only encrypted with the senders public key.

This patch converts the address objects to e-mail addresses before
passing them to the PGP library.