Email Error when checking out using SMTP email

I have a live store right now and it has been running fine using smtp.com for outgoing emails.
I've sent out multiple emails okay, but about an hour ago, I started getting this message: Email Error: The following From address failed: [email protected] (myzen cart store email) instead of the Checkout Success page

It's odd because the order confirmation emails actually go through to the intended destination, but instead of showing the checkout success page, I get the Email Error: The following From address failed: [email protected] Error.

Right now, I have the email settings back to PHP, even though I'm blacklisted on for some mail servers, but it seems to be working (but not through SMTP.com)


My settings:
Title Value Action
E-Mail Transport Method smtp
E-Mail Linefeeds LF Info
Use MIME HTML When Sending Emails false Info
Send E-Mails true Info
Email Archiving Active? false Info
E-Mail Friendly-Errors false Info
Email Address (Displayed to Contact you) xxx.com Info
Email Address (sent FROM) xxx.com Info
Emails must send from known domain? Yes Info
Email Admin Format? TEXT Info
Send Copy of Order Confirmation Emails To xxx.com Info
Send Copy of Create Account Emails To - Status 0 Info
Send Copy of Create Account Emails To xxx.com Info
Send Copy of Tell a Friend Emails To - Status 0 Info
Send Copy of Tell a Friend Emails To xxx.com Info
Send Copy of Customer GV Send Emails To - Status 0 Info
Send Copy of Customer GV Send Emails To xxx.com Info
Send Copy of Admin GV Mail Emails To - Status 0 Info
Send Copy of Customer Admin GV Mail Emails To xxx.com Info
Send Copy of Admin Discount Coupon Mail Emails To - Status 0 Info
Send Copy of Customer Admin Discount Coupon Mail Emails To xxx.com Info
Send Copy of Admin Orders Status Emails To - Status 0 Info
Send Copy of Admin Orders Status Emails To xxx.com Info
Send Notice of Pending Reviews Emails To - Status 1 Info
Send Notice of Pending Reviews Emails To xxx.com Info
Set "Contact Us" Email Dropdown List Info
Allow Guest To Tell A Friend false Info
Contact Us - Show Store Name and Address 0 Info
Send Low Stock Emails 0 Info
Send Low Stock Emails To xxx.com Info
Display "Newsletter Unsubscribe" Link? false Info
Audience-Select Count Display true Info
SMTP Email Account Mailbox [email protected] Info
SMTP Email Account Password Info
SMTP Email Mail Host smtp.com Info
SMTP Email Mail Server Port 2525 Info
Convert currencies for Text emails £,£:€,€ Info

http://www.zen-cart.com/forum/showthread.php?p=694079
This person has a similar error. See post 7.

Also: Even though I get the error message: Email Error: "The following From address failed: xx.xx.com," The order actually goes through!

Details for SMTP.com setup:
http://retail1.smtp.com/client_setup.htm

never mind.

Argh, sorry another clarification
When I receive that message
1. the order completes
2. emails do not get sent out.