Sometimes, you can trust major free email providers addresses, if they are personal friends. However, receiving business offers, or conducting business with these addresses should be avoided. Major free email providers include, but are not limited to – Yahoo.com, Hotmail.com, Msn.com, Aol.com, and Gmail.com.
You should avoid doing any business with business people with these addresses, and business people shouldn’t use them. They are sometimes OK for a personal account.
Another downfall with these addresses is that the are stolen quite often, especially Yahoo, Hotmail, and MSN.
You should also avoid contacting people who send emails from one address and want you to send it to a different one.
Internet service provider (ISP) addresses are a little more reliable, and own domain addresses are even more reliable. However addresses can be spoofed, and businesses with a domain are not necessarily trustworthy.
You may also want to avoid businesses that only have social media pages and not a regular web site. Especially if you don’t know them and the business doesn’t have an actual location.
As far as offers (email, phone, mail, magazine, newspaper) go, you may want to avoid them entirely. There are legitimate offers of course, that may help you. Be careful.
Update: There may be some people that use free email addresses that are legitimate emailing. However if they are soliciting you, it is still probably best to ignore. There have been complaints to agencies that have been legitimate, using free email addresses, and that is somewhat different than the point I’m trying to make in this post.