I can't find what's causing the error.
It's the message that BackupPC_nightly is trying to send at 1:00 am.
The msmtprc configuration file does not have user: "John". The only place I 
have "John" is in config.pl, at: $Conf{RsyncdUserName} = 'John';
Also, I have changed in BackupPC_sendEmail, in line 387:
 if ( !open(MAIL, "|$Conf{SendmailPath} -t $from") ) {
        printf("Can't run sendmail ($Conf{SendmailPath}): $!\n");
        return;
    }
I have added: "-t" since "msmtp -t" picks up the destination address from the 
e-mail "To:" header.

How can I debug this?

Note that this works fine:
/ # msmtp -d -t < /mnt/HD_a2/Temp/mailfile
Where mailfile is:
To: user_to AT domain1 DOT com
From: user_from AT domain2 DOT com
Subject: Mail test
This is a message
EOF
msmtp picks up the destination address from the "To:" header, and sends the 
message to: 
user_from AT domain2 DOT com

+----------------------------------------------------------------------
|This was sent by gh0988 AT hotmail DOT com via Backup Central.
|Forward SPAM to abuse AT backupcentral DOT com.
+----------------------------------------------------------------------



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