Make sure your username for the email account is correct (usually firstname.lastname@example.org)
Make sure that the SMTP server is correct (usually smtp.domain.com)
Make sure that the port you are using is correct.
Make sure that SSL/TLS selection is correct for your SMTP server.
Are you using a shared hosting SMTP server (for your domain) or are you using gmails hosted apps smtp?
Have you checked with your support folks at the hosting site to ascertain what the mailserver setup for SMTP should be (port #, SSL/TLS, server name, and login credential semantics)?
What hosting company is it?
If you want them to validate, then they should be able to have their email validation resent to them if I remember correctly. I can't remember if that is an add-on or core to tell you the truth. You can contact BlueHost and ask them to up the limits (I think they will go up to 750), and explain to them you run a forum and use email for validation.