[alzaabi]

I came here to correct some info.

by the way if you want the service to work
on vbulletin SMTP your site should be hosted with (VPS+WHM service)
OR ask the hoster to change or correct your nameservers,
then you can add all the records on your VPS cPanel ( delete the conflicted records and add Sendgrid Records) and whitelables the subdomain and the email in Sendgrid site
Finally go to "Setup Guide" in Sendgrid site and create the API Key , add these to vbulletin SMTP with your Sendgrid mail , all activation links will work.
and all should work.

Remember sending emails throught vbulletin mail box will be little slow on ALL SMTP services
but asking for activation links OR forgot your password links will be faster.