[horde] sending mail after upgrade fails

[Helga Mayer]

Hello list,

I recently ugraded from
php-5.2.6 -> php-5.2.11
imap2004e/c-client -> imap2007e/c-client
horde-webmail-1.2.3 -> horde-webmail-1.2.4

Now some users have problems sending mail.
postfix runs locally

In the horde logfile I get the follwing error message:
HORDE [error] [imp] Failed to connect to localhost:25 [SMTP: HELO was  
not accepted:  (code: -1, response: )] [pid 19899 on line 747 of  
"/horde-webmail-1.2.4/imp/lib/Compose.php"]

But there is no correspondent error message in the postfix log file.
No rejects at all due to the helo command.

postconf says:
smtpd_helo_required = no
smtpd_helo_restrictions =
strict_rfc821_envelopes = no

I've no idea if this is a php problem, a PEAR problem oder an imp problem.
The configuration didn't change, at least i did not intend to change
anything.
I'm running a test server with the same software versions. Strange enough
people who couldn't send email from the production server succeeded to
send email from test server. Users have different options on both servers.
Any hint where to look in horde-prefs perhaps?

Thanks in advance,
Helga Mayer