E-mail invitations don't send

For the past few months, when I send gallery invitations to clients, they very rarely receive them (maybe 1 out of 20 clients does). And although I check the box to also receive the e-mail invites, I don't get them, either.

I've tried resending over and over, and sending them to myself as if I were a client....no luck.

I always eventually give up and have to either burn DVDs and mail them to clients or, if they prefer, I use DropBox or my FTP site (most clients aren't comfortable with either of those).

As sharing galleries with clients is the main reason I use Photoshelter, I'd love to figure this out. Thoughts?

Thanks!