Contrary to what you might think, there is one case where it's better to send a series of emails to one person, rather than cram everything you have to say into one message.
Multiple, short emails can sometime create less overload than a single large email, and can be much more efficient. When is this the case? When you're delegating multiple unrelated tasks or asking multiple unrelated questions which require more than a one-sentence answer.
By writing separate emails, you can keep each message short and to the point, which makes it easy for the recipient to read and understand what you want. More importantly, each message gets its own subject line, which you can now optimize to fit the task at hand.
What's better? Compound messages such as these:
Subject: I need you to do a few things
Subject: Need your input
Or focused messages such as these:
Subject: Interview John Smith, shortlisted candidate for Support Manager
Subject: Finalize travel arrangements to next week's conference
Subject: Vista schedule slippage: Delay to 2007 or cut features?
Subject: Away next week: Can I review your draft proposal beforehand?
Many people use their inbox as a to-do list, and file or delete each message as they complete the related work. So if you're delegating tasks, make the subject sound like a task! If you're asking a question, phrase the subject as a question!
If you bury multiple tasks in the same message, you divide the attention given to each one. None of the tasks is clearly visible in the list of messages in the inbox, and the recipient will have to keep your message around and re-read it every time she scans her inbox to discover which tasks she has not yet completed. There's a good chance she'll miss something — you are setting her up for failure.
By using separate messages, you are ensuring that each task gets the attention it deserves.
Remember: this works only for unrelated tasks — people will not take kindly to this if the tasks are closely related.
P.S. This also helps the compulsive filers among us — we can file each message in a separate folder, instead of agonizing over which folder is the best one for the compound message! :-)