Try changing "Conditions tab: BGP4-MIB:bgpTraps.0.1 contains *established*" to "Conditions tab: BGP4-MIB:bgpTraps.0.1 is equal to *", that way you should get an email for all the BGP traps that come in. See if that works. Also check if you have you SMTP Server details filled in in the Edit E-Mail/Page Action section. Within the Trap Viewer rule, you have to set what the SMTP server is.
I use Syslog Viewer instead to alert me of BGP neighbour changes.
For Message Type Pattern I look for BGP-5-ADJCHANGE
Then in the email message I send
Subject: BGP Neighbour Change on ${Node.Caption}
Message: ${MESSAGE}
Hope that helps.