Unable to queue message

I'm using qpsmtpd 0.31.1, and some of my users (using Outlook Express)
receive the error "Unable to queue message" immediately upon attempting
to send a message to a non-local domain (typically it's a text page to
vtext.com).

Anyone know why this would be happening? Restarting qpsmtpd usually
fixes the problem--they just need to trash the original message and the
new one goes out fine.