Emailing can be a tricky form of communication. There are mistakes to be made. I have made more than a few of them. I hope this can help you avoid some of the same mistakes.
1. Using a non-descriptive headline.
A “Hi!” may be nice to start a regular conversation. But when we are talking about emailing and subject lines then a more descriptive subject line is useful. It will quickly convey what your email is about and it will stand out among the other 100 subject lines your reader is scanning through in his/her inbox.
2. Rambling on and on.
It's easy to fall into the trap of wanting to talk on and on about your problem, question, new product etc. It's great or interesting, so why not? The problem is just that people get a lot of emails. Maybe dozens or hundreds each day. If you don't get to the point quickly then you may lose their interest halfway through the email. So, get to the point quickly.
3. Not explaining properly.
It's easy to assume that people know as much as you do and that some things are just self-evident. That's not always the case though. You shouldn't write too long and rambling emails. But you shouldn't write too short emails either. If you have something that may need to be explained, explain it clearly but as simply as you can. Make sure that you get the point of the email across to your reader.
4. Being too formal.
Having a really formal tone can be helpful sometimes. Other times a less formal tone is more useful.
An email is often a way to build and develop a relationship of some sort. To do that, emotions must come into play. You want to create an emotional connection to your reader. That doesn't mean that you should be overemotional though.
Just try to write to someone in a relaxed tone. The same tone that you would use if you met that person in a relaxed conversation in real life. Write kinda like you talk. Then your email will have a conversational tone. And the connection between the two of you will often be better and more natural. How do you write in a conversational tone? Be relaxed when you write the email and your tone will be relaxed. If you are tense or anxious, try a few belly breaths or take a short break out in the fresh air.
5. Messing up your contact information.
Have you included all the necessary information on how to get a hold of you? What is your phone number or fax number? When are you available for calls? What is most likely the quickest way to get a hold of you? And, finally, don't forget to triple check the addresses and digits for your website, phone etc.
6. Using email in the first place.
When phone or face to face is better. There is certainly a larger risk for misunderstandings if you use email. There is no body language or voice tonality to convey nuances and emotions. And emoticons can only go so far. :) If you have a possibly emotional issue to bring up consider using the phone or meeting face to face instead. No point in creating totally unnecessary conflicts.
7. Not reading through your email an extra time before hitting send.
There may be typos, spelling errors or even things you thought about writing but forgot too add. Again, thoroughly check your contact information in the email. And don't forget to check that your attachment is actually attached to the email.