Your emails make an impression on your managers, colleagues, clients, and on possible recruiters. Sloppy emails may show that you are not disciplined and risk causing misunderstanding among colleagues. A poorly-written email along with a job application can damage your chances of landing the job.
Emails can be easily forwarded and thus be read by more people than you think. So re-read your emails carefully before sending it, to make sure that it is not offensive and that it doesn't say anything that could get you into trouble.
How quickly you need to reply to an email typically depends on the nature of the email. In general, you should reply immediately. If you know you don't have an immediate answer to a particular query, reply to acknowledge the email and give the person a time frame of when you think you'll be able to respond.
It might be tempting to call up the person you just emailed, but desist. It can be annoying for the recipient.
In general, avoid sending large files as attachments since they clog up the recipient's inbox.
Don't send emails for every little thing and especially not for something that can be tackled easily over the phone or in person. 'Emails are there to save your time, not waste it,' says Mr. Singh of Panasonic.