SMTP Error: Data not accepted during checkout

-- HikaShop version -- : 2.6.1
-- Joomla version -- : 3.4.8
-- PHP version -- : 5.4.45
-- Browser(s) name and version -- : multiple
-- Error-message(debug-mod must be tuned on) -- : SMTP Error: data not accepted.

When going through the process of checkout we are now getting an error:

SMTP Error: data not accepted.

At the END step in the process.

The admin notices ARE being sent, but the email notices to customers are NOT being sent.

The core Joomla email system IS working.

The HS outbound email was working fine until about 24 hours ago, then we started getting this error.
We were originally using 2.6.0 (when the error started). I upgraded to 2.6.1 since then however the issue has not resolved or changed.

We are using Amazon SES for email delivery and have not received any notices from them related to any problems with sending.

Any ideas - or where to check for issues?

NOTE: We often have 'strange' problems like this with email sending from HS. It would sure be nice to have some sort of 'test button' to test connection from within hikashop to try to resolve issues like this.

OK ... after checking one other area I found the solution (for those having similar issues).

We had modified the templates for email and set the email FROM address to an email address we were using then, but not the one that we are presently using (i.e. the login account under SES).
Evidently SES must have just changed the account security to only allow sending FROM the account with access.
Simply changing that seems to have resolved this issue.