I've noticed it frequently over the past year, a lot of people suck at email.
Before I begin this post, I must give credit where credit is due (Ainc rule #2) and acknowledge that a lot of this content comes from David Cohen's chapter in the book Do More Faster – "Don't Suck at E-mail". Please read it here.
Like it or not, a large portion of the communication done in most jobs is via email. You owe it to yourself and those you correspond with to not suck at it. Follow these three guidelines and you'll be most of the way to not sucking at email.
1. Actually read it and respond – I can't tell you how many times I'll send an email to someone and then see them a few weeks later only to have them ask me a question I've already answered. I want to respond with, "the answer is sitting in your inbox." So, step one is to actually read and retain the information from the email. If the message merits a response, give one. Don't leave the sender hanging wondering if their message got lost in the Bermuda triangle of the nets. Give that poor soul an answer. Even if it's one you think they don't want to hear.
2. Reply All (when relevant) – If someone copies a colleague on an email message, it was likely on purpose. Keep that person on the thread unless there is a good reason not to. This often happens when you're trying to coordinate with three or more people and one person continuously hits the reply button instead of reply all resulting in only one person getting the information that was intended for the entire group.
There are obviously exceptions to this – namely, if someone is giving you an intro to a third party, kindly move the introducer to the bcc field with a note that says, "Thanks for the intro. Moving you to bcc." You can read more advice on this from our friends at Techstars in their book Venture Deals.
3. Be concise – Most emails can be summed up in three sentences or less. Keep your emails shorter and you'll be more likely to get quicker responses.
Other tips:
1. Use it as a branding opportunity (great tip from David's post) – always send email from your company email address and add a signature line to the bottom. I use this as the signature line for all my emails:

2. Respond quickly (2 day max) – This is related to guideline number 1 above, but always try to respond within 2 days to emails. Even if you don't have the complete response ready, at least respond and say that you received the email and are working on it.
3. Use the subject line – Emails that end up in my inbox with subject line "(no subject)" are a sure sign of an amateur. Don't make this rookie mistake. Make the subject line short and relevant.
4. Proofread – Use spellcheck and review your emails before you send them. This is especially important if you are sending something to a potential client, investor, or partner. They will judge you by your email content.
5. Get a good junk mail filter system – In order to help out with guideline number 1 above and have the capability to Read and Respond to all emails, you need to avoid receiving junk mail. Most email servers are pretty good at this and can be trained to move junk mail to a junk mail folder. Gmail is exceptional at this. Invest some time in ensuring you have good junk mail filter system.