Emails being blocked with 421 - throttling issue?

Hello,

I have issues sending emails to several of my customers which are using messagelabs.com email edge. I'm getting 421 Timeouts when sending messages from two of my edge servers. What is the proper way to have my IP addresses removed from throttling on your end?

IPs in question are 188.114.87.134 & 91.150.167.126

Thanks!