MarketPress gives me an error message

Orders are properly recorded. The card is properly charged. Both of the emails are properly delivered. Problem is... the customer sees an error message on this page /store/shopping-cart/confirm-checkout/

Client uses Gmail as the outgoing mail server and so I installed 'Easy WP SMTP'. The message seems to involve Google, but because the emails are successfully delivered I am at a loss to understand how to get rid of this error message:

SMTP -> FROM SERVER:220 mx.google.com ESMTP z5sm30463318obg.13 - gsmtp

SMTP -> FROM SERVER: 250-mx.google.com at your service, [50.87.152.103]
250-SIZE 35882577
250-8BITMIME
250-AUTH LOGIN PLAIN XOAUTH XOAUTH2 PLAIN-CLIENTTOKEN
250-ENHANCEDSTATUSCODES
250-PIPELINING
250 CHUNKING

SMTP -> FROM SERVER:250 2.1.0 OK z5sm30463318obg.13 - gsmtp

SMTP -> FROM SERVER:250 2.1.5 OK z5sm30463318obg.13 - gsmtp

SMTP -> FROM SERVER:354 Go ahead z5sm30463318obg.13 - gsmtp

SMTP -> FROM SERVER:250 2.0.0 OK 1393722289 z5sm30463318obg.13 - gsmtp

SMTP -> FROM SERVER:221 2.0.0 closing connection z5sm30463318obg.13 - gsmtp

SMTP -> FROM SERVER:220 mx.google.com ESMTP x3sm43525615oek.3 - gsmtp

SMTP -> FROM SERVER: 250-mx.google.com at your service, [50.87.152.103]
250-SIZE 35882577
250-8BITMIME
250-AUTH LOGIN PLAIN XOAUTH XOAUTH2 PLAIN-CLIENTTOKEN
250-ENHANCEDSTATUSCODES
250-PIPELINING
250 CHUNKING

SMTP -> FROM SERVER:250 2.1.0 OK x3sm43525615oek.3 - gsmtp

SMTP -> FROM SERVER:250 2.1.5 OK x3sm43525615oek.3 - gsmtp

SMTP -> FROM SERVER:354 Go ahead x3sm43525615oek.3 - gsmtp

SMTP -> FROM SERVER:250 2.0.0 OK 1393722292 x3sm43525615oek.3 - gsmtp

SMTP -> FROM SERVER:221 2.0.0 closing connection x3sm43525615oek.3 - gsmtp

<b>Warning</b>: Cannot modify header information - headers already sent by (output started at /home/napkins/public_html/wp-includes/class-smtp.php:121) in <b>/home/napkins/public_html/wp-includes/pluggable.php</b> on line <b>896</b>