Socket Error when sending mail from Webmail (Horde, etc.)

[tiggerific]

I get the following error when trying to send mail from Horde or other webmail programs:

There was an error sending your message: Failed to set sender: email@email.com [SMTP: Failed to write to socket: not connected (code: -1, response: )]

I have searched and found two suggestions - one is to enable SMTP_ALLOWLOCAL in CSF (which was already enabled with a "1"). The other suggestion was to ensure the Horde Identity had a valid email from address. I set the email address and this still has not resovled the problem.

I tried completely disabling CSF to see if it was something caused by the firewall, but that hasn't helped.
I have done a cPanel upgraded, checked SMTP settings in Tweak Settings and checked Security levels, but nothing has resolved this.

Has anybody got an idea where I should look next?

[Infopro]

Are you logged into Webmail as the cPanel user, or the email account user?

[cPanelTristan]

Was MySQL upgraded or downgraded recently on the machine? There are reports in our ticket system where a MySQL downgrade caused this to occur with Roundcube. I'm trying to determine if this might be the same cause.