After POA upgrade to version 5.5.x Horde Webmail is no longer working. It is impossible to send an email, you just receive the following error message:
There was an error sending your message: unable to add recipient [firstname.lastname@example.org]: Invalid response code received from server
/usr/local/qmail/shared/control/whitelist is missing on the mail servers in the QMail cluster or the file exists, but its contents does not have the Frontnet IP of the Webmail server.
An entry for the Frontnet IP of the Webmail server should be added to the
/usr/local/qmail/shared/control/whitelist file like below:
[root@linmail01 ~]# grep 10.10.10.10 /usr/local/qmail/shared/control/whitelist 10.10.10.10/32
Where 10.10.10.10 is the Frontnet IP of the Webmail server.