Please note that Yahoo/AOL/Verizon are more likely to put your emails in their spam boxes or delay delivery with no reason given. They have their own way of doing things and it's a dance at best to get them to allow emails to the inbox on a consistent basis. Other smaller email providers have this issue with them as well. Email delivery is a constant battle because of people who want to use other's services for no good.
Other than that, please keep reading:
There are a couple of reasons why your messages would go to the spam folder when you send to someone who uses another email provider. Here are a few ways to help make sure that your emails go into the inbox and get seen.
1. This one should be pretty obvious: Do not send unsolicited email. Please do not send out multiple emails to people that you do not know or send all your newsletters and articles that you find interesting to everyone in your contacts. They may not want your forwards or newsletters. Please ask first. People can and will report you for spam instead of asking you to stop.
2. Always make sure you are sending emails through our server (make sure to use our outgoing/smtp servers in your settings or send through webmail) and are authenticating with your email address and password. If you use our webmail, it's automatically sent through our server. If you use an app or email client to send emails, you should make sure you are using our smtp information and not your internet provider's information. Also, emails sent through other providers' webmail such as gmail and outlook.com may get marked as spam so keep an eye on that if you send through another provider.
3. This one is important: Ask your contacts to put your email address in their address book / contacts. That should bypass the spam filter at their provider and place your emails in their inbox.