You're limited in your ability to control SPAM, as different email providers have different rules and restrictions. From our experience, however, it's best that you use your own institute email address (e.g.
sisterhood@beth-el123.org) to send your emails
from vs. Yahoo and AOL email addresses which seem to be the most problematic (gmail has been less restrictive, i.e. a better option, if you want to go with one of the common email providers).