What are the best email practices that influence delivery?

The best email marketing practices can be summarized into 4 points:

•    Acquire your list properly, do not add any addresses that did not express explicit permission to be mailed by you! Double opt-in is a good choice.
•    Provide your subscribers with valuable content, and send them the messages that they expect. This guarantees proper list engagement because subscribers will be eager to read your mailings and will not report them as spam.
•    Do not “flood” your list with too many messages. If you see lower engagement – take some preventative steps –  and you can determine which subscribers are no longer interested and try to win them back. If they’re still unresponsive – let them go.
•    Respect your subscribers – process all unsubscribe requests and listen to your recipients’ suggestions.
•  Do not use Yahoo!, aol, aim From email address, this will cause every message to bounce.

You can also use Spam score checker available in the message editor to review the content of your message to find out whether anything may cause it to be classified as spam.