The connection over the e-mail client (like Outlook) is broken intermittently either when sending mail or just within the connection to the server. One of the possible errors below:
CONFIG_TEXT: Failed to send some messages - javax.net.ssl.SSLProtocolExpeption:SSL handshake aborted: ssl=0[7a4fb1db08: Failure in SSL library, usually a protocol error
Qmail is used along with the DNSBL.
This issue is caused by the Plesk bug with ID #PPPM-11223. It is going to be fixed in future updates.
As for now, the possible workarounds are: