[ic] Unable to send mail using Net::SMTP IC 5.4

[Steve Graham]

Hello,

IC 5.4 using Net::SMTP

from catalog.cfg:
# Set the mail handler
SendMailProgram Net::SMTP
Variable  MV_SMTPHOST  my.mailer.com

IC is sending the initial email to the customer, and to the order 
processing email address without any issues.

When the order is filled, and processed on the backend  (ship & 
notify customer) is when the error log shows the error message:
Unable to send mail using Net::SMTP


Any suggestions where to look?

Thanks,
Steve