Intermittent SMTP error: Failed to transport message. Error during SSL handshake: error:1408F10B:SSL

I intermittently get an SMTP error of: "Failed to transport message. Error during SSL handshake: error:1408F10B:SSL routines:ssl3_get_record:wrong version number" when sending mail from an e-mail client configured to send via my ISPConfig 3 server configured using port 587/STARTTLS.

The annoying part is that it's intermittent. Some while later (a few minutes to a few hours) the mail will send using the exact same settings.

Can someone whack me with a clue-bat and gimme a clue as to what's going on? TIA.