Many people, myself included are offended by strangers being over-familiar. For example I react badly to people I don't know addressing me as 'Dave'. Only friends and family call me that. As a rule, use the title or form of address that you would use in verbal communication. Keep download size to a minimum .
Big graphics can make e-mails take a long time to load.
If you have an attached file, the recipient will often have to wait for your full message to load before they can retrieve it. Its irritating to wait many minutes for a message to load only to find out the attached page was not worth the wait. But for those so inclined, not only are these likely to offend the recipient, people found engaging in illegal activities involving email are likely to have strong sanctions brought against them by the university and by the civil authorities. Email is not confidential . Its almost laughably easy for the contents of your email to be read by others without your knowledge.