[horde] SMTP Failing to send email

[Louis-Philippe Allard]

For weeks my horde setup has been running fine even this morning I was
sending emails when suddenly I tried to send an email and got a red popup
saying:

  There was an error sending your message: Failed to send data [SMTP:
Invalid response code received from server (Code: -1, response: )]

  And the email was never sent.  I retried several times, all failing.

  Whats happening ?