GreenTree's outgoing (SMTP) servers require your email client to authenticate before you can send email. If you don't authenticate, the server will return a "553 needs authentication" error message to your email client and refuse to accept your email.
See How do I enable SMTP Authentication? for more information.

Email error messages usually mean what they say. For example, if your bounce message says "Email box is full," the mailbox of the person you're trying to contact is probably full. If the message says "Invalid TO: header," then there is probably something wrong with what's been entered in the TO: field, etc. Take a good look at the headers in your bounce message to see if you can spot the problem before asking for help or assuming something is broken. Most email problems are simple typos or are caused by someone putting something that's not a real email address in the TO:, CC: or FROM: address fields.

AOL (and several other ISPs) have blocked access to port 25 except for their own servers. This is to prevent their members from sending spam directly to external servers. You can try to set your client to use mail.greentree.ca port 2525 instead of port 25, and you might be able to get through. Otherwise you may choose to send mail through your ISP's mail server, and receive through your greentree account on the greentree server.

Your email address must match the server you use to send your email. If you are sending email with your email address set to @home.com, then your SMTP server should be your home.com server. If your email address is set to your GreenTree address, then you should use mail.greentreewebhosting.com as your SMTP server.
N.B. You may set your reply-to address to be anything you want. For example, if your email address here is fred@yourdomain.com and you want to use GreenTree's mail servers, you might set your reply-to address to wilma@flintstones.org. This way anyone who responds to your letter will write to wilma@flinstones.org, and your outgoing email will work correctly, too.

You have your email address set to your GreenTree address, but are using your ISP's servers to send mail. This is one of the more common types of the famous "forged header" problem with email. In short, you are trying to lie to the email system. If you say you are "so-and-so@yourdomain.com", but are actually coming from aol.com or home.com, most servers will think you are spamming (since that's what spammers do). On systems where such spam quietly disappears, your email will simply vanish. On other systems, your email may or may not make it through. One thing's certain - if you send it here addressed this way, it _definitely_ won't get through. Fortunately for you our servers usually catch this kind of thing and inform you so it can be corrected by you.

The domain portion (the part after the @ sign) of your email address should always match the domain of the service you are using to send the email. If you are using GreenTree's servers, use your @yourdomain.com email address. If you are using myisp.com's servers, use your @myisp.com email address. Even if your email seems to go out okay without following this rule, chances are good it won't make it to its destination. You may or may not get a bounce message depending on how the various servers through which your email passes are configured.

There's another good reason for this rule -- imagine if anyone anywhere in the world could send email out with YOUR name on it. GreenTree's servers prevent this by insisting that anyone who claims to be sending email from GreenTree prove his/her identity by authenticating. It won't be long before all legitimate email services require this. More do every day.