SMTP errors on outbound mail

[catalano 1]

I recently switched from using Sendmail to SMTP mail and now any mail sent to addresses at major ISPs are being bounced with the following error:

SMTP error from remote mail server after end of data:
    550 Messages should have one or no To headers, not 2.

I've looked at the source code of the outbound messages and there are in fact 2 "To" lines in the message. I've seen a few other posts about this going back to 2012 and I don't see any concrete solution. Was anything ever done about this?

Thanks.