The following is the list of Yammer ports for access to the service:

Ports: HTTP/S on 80/443 only

The following URL’s for proxy access are on top of the standard O365 portal access ones:

*.assets-yammer.com
*.yammer.com
*.yammerusercontent.com
ajax.googleapis.com
*.cloudfront.net
crl.microsoft.com
evsecure-ocsp.verisign.com
evsecure-aia.verisign.com
evsecure-crl.verisign.com
sa.symcb.com
sd.symcb.com
*.omniroot.com
*.verisign.com
*.symcb.com
*.symcd.com
*.verisign.net
*.geotrust.com
*.entrust.net
*.public-trust.com

and the following is the IP address range for the service: 134.170.148.0/22

Finally ensure certificate revocation is allowed!