Need immediate help

Before sending emails to 2000 members you need to check with your web host that you are allowed to do this, most web hosts have a bulk email policy and will kick you if you attempt to send this amount of emails. This could also explain the error you get as well.

As to the issue of your members not getting any emails, if your members don't get an error then it means that the forum software is doing it's job and relaying on the email to the SMTP server.

This will mean that the problem is probably with your web hosts SMTP server. They may have blocked you form using it due to you sending bulk email, or they may have policies in place that you can only relay on emails to local email address.

Allot of hosts have these types of policies to prevent spammers using their servers, you will need to contact your web host about this.