One point Per Email:

One of the advantages of email compared with traditional letters is that it doesn't cost any more to send several emails than it does to send one. So, if you need to communicate with someone about a number of different things, you should  write a separate email on each subject.
That way, your correspondent can reply to each one individually and in the appropriate time frame. One topic might only require a short reply, that he or she can send straight away. Another topic might require more research. By writing separate messages, you should get clearer answers, while helping other people manage their inboxes better.

Good Example:

Subject: Revisions For Sales Report
Hi Jackie,
Thanks for sending in that report last week. I read through it yesterday and feel that you need more specific information regarding our sales figures in Chapter 2. I also felt that the tone could be a bit more formal. The report is going to be read by our Executive Team, and needs to reflect our professionalism.
Thanks for your hard work on this!
Monica

AND

Subject: Friday 10/9, 11am Meeting w/PR Dept
Hi Jackie,
I wanted to let you know that I've scheduled a meeting with the PR department for this Friday, 10/9, regarding the new ad campaign.
It's at 11:00am, and will be in the small conference room. Please let me know if you can make that time.
Thanks!
Monica

By separating those two important communications, Jackie will be able to find what she needs quickly in her inbox. As well as this, separating the two topics helps her keep her saved emails relevant. Once she's done with the revisions email she can delete it, but keep the meeting reminder email until the end of the week.

Bad Example:

Subject: Revisions For Sales Report
Hi Jackie,
Thanks for sending in that report last week. I read through it yesterday and feel that you need more specific information regarding our sales figures in Chapter 2. I also felt that the tone could be a bit more formal. The report is going to be read by our Executive Team, and needs to reflect our professionalism.
Also, I wanted to let you know that I've scheduled a meeting with the PR department for this Friday, regarding the new ad campaign. It's at 11:00, and will be in the small conference room.
Please let me know if you can make that time.
Thanks!
Monica

In one email she put two different topics so its not very clear to the reader which one is more important.