We are using Mynxx and have only just turned guest checkout on.
We keep getting error messages.
Have attached a copy of the error we get back.
Appears to be sendng to: To: =?u\-8?B?R3Vlc3Q=?= <>
We don't touch the email functionality or the checkout functionality at all, so this won't be anything theme related. I also haven't seen this come up before, so can't offer any advice I'm afraid. I'd suggest you try either Googling the error or posting on the official Magento forum.
STEP 1 IN MAGENTO DEVELOPMENT - DISABLE YOUR CACHE OR YOU WON'T SEE ANY CHANGES.
Use the new "secure" tab when replying for privately sharing links.
Check the
"Useful Snippets"
post for tips.