Sometimes when macs update, SSL on email accounts gets enabled by default. Our server doesn't support SSL so you may need to disable this,
Please try the following:
- Open Apple Mail.
- Click the Mail menu and select Preferences.
- Select your mail account on the left under Accounts, then click the Advanced tab.
- Confirm the check box labeled "use SSL" is not checked next to ports. If necessary remove the checkmark.
- Click the Account Information tab and select Edit Server list from the drop down next to Outgoing Mail Server.
- Click the Advanced tab and confirm there is not a checkmark next to Use Secure Socket Layer(SSL).
- Click OK and close the accounts. Window and choose to save.
- Click Save to update your settings.