Error: Unable to send e-mail. Please contact the site admin if the problem persists.

I'm using u create together with Organic Groups on Drupal 6. I want to send an email message to invite users to join the group. I am using a hosting service to test my application.

When I send an invitation through the u create form, I receive the following error message in through the form:

Unable to send e-mail. Please contact the site admin if the problem persists.

And the following error appears in the Watchdog window:

Error sending e-mail (from xyz@netvision.net.il to [Funny Ideas] We have created an account for you).

It appears that the correct "from" email address is used, but the wrong "to" email address is used, and it is picking up what I defined as the site name instead. (Obviously that is incorrect, since it should be mailing the mail message to the organic group invitee.)

It seems to me that sendmail is configured correctly on my hosting service. Registering new users to the site is successfully achieved as well.

Could it be a configuration problem or a problem of something that I've missed? Should I be using SMTP or some other module instead?

Thanks for any help,
Momo