There are times when connecting to Apple Mail using Exchange Web Services fails. The most common cause of this is the connection type that Apple Mail attempts, by default. However, there is a simple solution to this issue.
By default, Apple Mail will attempt a secure connection to a mail server. That connection is either attempted using SSL or TLS. However, not every email configuration uses a secure connection. (However, they should!)
To allow Apple Mail to connect to your mail account using EWS, you have 2 options:
1. Purchase a SSL certificate and have your system administrator install the certificate on your mail server. This is the recommended solution as it's the safest and most secure way to connect to your mail account.
2. Alternatively, you can change the connection type in Apple Mail to allow for insecure connections. While NOT recommended, there may be instances where this is your only recourse.
First off, you can tell if there is a problem with the account you set up in Apple Mail connecting to SmarterMail pretty easily: once your account is set up, a error is thrown by Apple Mail. (Denoted with a pop up and/or an exclamation point (!) showing up next to the connection type in your mailboxes list in Apple Mail.)
If you decide to bypass the security defaults Apple Mail uses, Open up the Preferences area within Apple Mail. If it's not already selected, click on the Accounts tab. You'll see your SmarterMail account listed, generally denoted as an Exchange account. On the Accounts page, click the Server Settings button.
Here, you'll see the basic settings for your SmarterMail account. To modify the connection type, click on the Advanced Exchange Settings button. When that modal pops up, you'll see a checkbox: "Allow insecure authentication." Check that box and hit OK. Be sure to click the Save button on the Server Settings page.
NOTE: When setting up Apple Mail to use EWS, you MUST make sure you're using the proper Internal and External URLs for your mailbox. These will be similar to: http://mail.example,com/ews/exchange.asmx. If you do NOT use these URLs, your connection will always fail, regardless of how you have security configured for your account.
Once the security change is made, you should be good to go. As mentioned, however, using a secure connection for your mail server, whether that's use SSL or TLS, is highly recommended.