Horde – Error on sending email

I run Horde 5 on one of my servers as webmail/groupware client. I recently migrated that server to a new environment (Ubuntu 14 LTS) and reinstalled Horde5 using pear.. After installation everything seemed to work fine, except sending emails. The error my client saw was “Error communicating with server”.. After analysing the logs I found this error:

The solution was quite easy but hard to find; after an upgrade of Pear a version of Horde_Mime was installed that borked the sending of email.. Installing an old version with pear did the trick;