All marketing needs to inform all marketing as overarching marketing optimization will always produce more significant benefits than channel by channel tinkering. Even in apparently disparate fields such as search engine marketing (acquisition) and email (more readily associated with retention). Marketing insights garnered from one of these channels can readily be applied to inform and optimize the other.
Using the language of intent (the keywords used in a search - both in an engine and on your site) better informs the content and, more specifically, the subject line of your emails. If a specific set of keywords are driving traffic to your site, use them to increase your email open rates. PPC also gives you a great rapid cycle testing arena for your offers, include the best performing (and most profitable) in your email campaigns.
In terms of timing, if you see that the peak period for traffic to a particular service or product on your site is between 6PM and 10PM, time your email promotions to arrive during the same period. Compare results against a control group.
A study last year by Datran Media demonstrated that more than 70% of marketers see email as a complimentary channel to search, I have yet to see evidence of the two direct response channels being used in harmony to deliver the right message at the right time...