ERROR: Failed sending email / SMTP Error

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]

If it worked before and you are having a problem now and you didn't make any changes, then you need to talk to your host and find out what they changed.

Nope they don't seem to have a clue why its happening. Do you know if the exact error is being stored anywhere? The host has disabled the mail() function, but we have been using SMTP Auth the entire time.

Just for reference, this problem isn't due to zen cart, its something to do with the SMTP mailer class / smtp server.

The problem seems intermittent, it wont work for a few minutes, then after leaving it for a while works again.

That error message is triggered by a response from your SMTP server itself, specifically the SMTP server has rejected the recipient address. You need to work with the administrator of the SMTP server to find out why the SMTP server is rejecting those addresses.