Sending an email triggers a HTTP 500 error

Anytime I try to send mail - from the admin "send email" function, from the store using the the forgot password link and most importantly, comfirmation emails, I receive HTTP 500 Internal Server Error!

Any insight would be helpful - I've been at this for some time.
Zen Cart 1.3.8a, db 1.3.8

My email settings are as follows:
E-Mail Options

Title Value Action
E-Mail Transport Method smtpauth
E-Mail Linefeeds CRLF
Use MIME HTML When Sending Emails false
Send E-Mails true
Verify E-Mail Addresses Through DNS false
Email Archiving Active? false
E-Mail Friendly-Errors false
Email Address (Displayed to Contact you) [email protected]
Email Address (sent FROM) [email protected]
Emails must send from known domain? Yes
Email Admin Format? TEXT
Send Copy of Order Confirmation Emails To [email protected]
Send Copy of Create Account Emails To - Status 0
Send Copy of Create Account Emails To [email protected]
Send Copy of Tell a Friend Emails To - Status 0
Send Copy of Tell a Friend Emails To [email protected]
Send Copy of Customer GV Send Emails To - Status 0
Send Copy of Customer GV Send Emails To [email protected]
Send Copy of Admin GV Mail Emails To - Status 0
Send Copy of Customer Admin GV Mail Emails To [email protected]
Send Copy of Admin Discount Coupon Mail Emails To - Status 0
Send Copy of Customer Admin Discount Coupon Mail Emails To [email protected]
Send Copy of Admin Orders Status Emails To - Status 0
Send Copy of Admin Orders Status Emails To [email protected]
Send Notice of Pending Reviews Emails To - Status 0
Send Notice of Pending Reviews Emails To [email protected]
Set "Contact Us" Email Dropdown List
Allow Guest To Tell A Friend false
Contact Us - Show Store Name and Address 1
Send Low Stock Emails 0
Send Low Stock Emails To [email protected]
Display "Newsletter Unsubscribe" Link? true
Audience-Select Count Display true
SMTP Email Account Mailbox [email protected]
SMTP Email Account Password *********
SMTP Email Mail Host 72.15.228.171
SMTP Email Mail Server Port 25
Convert currencies for Text emails £,£:€,€