Not sure, exactly, but I would suggest looking at your SMTP connection, and see if yours requires SSL engaged ? Test with turning it off, briefly?
I do vaguely remember reading something about PHP configuration changes -- my search of the forum turned up this url, and others, which might start you on your way:
www.limesurvey.org/en/forum/installation...il-invitations#75826
Suggest, too, that as you address this, that you let on which version of LimeSurvey you're using, and your PHP with your ISP as it might make a difference to someone familiar with the detail of email sending.