Are you offering web design to web agencies? The 1st question is if they outsource already. If they do you have a better chance to win the biz. If they don’t you need to stand out from the million emails we receive with something funny, entertaining, valuable to start a conversation.
Agencies want less headache, white label work, quality of work, be prompt, clear grammar and communication, knowledgeable, know of problems before they arise and get shit done.