Invite module not sending emails

Hi,

We have the Invite module installed on a site. The problem is that when a user clicks on "Send invite" nothing happens. The site is configured to use MIME mail. All other system emails (e.g. for new friend requests) are OK. Users have permissions set to send invites for this module. The Invite module config settings look OK. But no email is sent, and no "Message sent" appears on the page after "Send invite" is clicked. Is there something that has to be set to allow the emails out? Thanks.