I never had a problem with this for about 10 months previously, but now i keep getting this error at the top of the page when updating customer orders. It happens prob around 1 in 5 customers, and with a few retry it works.

The email addresses arnt black listed, are valid, and are in the valid format.

ERROR: Failed sending email to: "XXX XXX" with subject: "Order Update #XXX" SMTP Error: The following recipients failed: [email protected]