I did find one 2 year old thread (http://www.zen-cart.com/forum/showthread.php?t=50244) about this, but there was no resolution.

When sending an email from the GV section, the dropdown selector doesn't send the email, but entering a single email address in the "eMail to" field works as expected.

The cart is 1.3.7 - I have been able to test on another 1.3.7 cart and that works perfectly. Any suggestions as to where I should start looking for the solution?