Why do you need to do this?
You can send email through Radius for direct messages with templates, date triggered and other automated emails and appointment reminders. To do so, you need to set up your email connection so Radius can send emails through your email provider on your behalf.

If you are the ACCOUNT OWNER, Go to Settings > EMAIL, COLUMNS, CALENDAR & OTHER SETTINGS. Check Activate SMTP and connect your settings (see below). Submit Settings. This will become the default email address if an agent hasn't set up their SMTP or if the lead/client doesn't have an agent assigned to them.

If you are NOT the account owner (another admin or agent), the account owner must set their SMTP up first. Then, you'll go to Account> My Information> Email. Check Activate SMTP and connect your settings (see below). Submit Settings. When you're the assigned agent, your email address will be used for the sender. If you're sending a message directly to a lead/client (as opposed to it being an automated email), the email will generate from whomever is logged into their profile and sending that message.

Configure Gmail Email
Click on the "Connect Gmail" button. Follow these steps to learn more if you need them.

Configure Office 365 Email
Click on the "Connect Office365" button.

Configure Yahoo Mail
SMTP Host = smtp.mail.yahoo.com
SMTP Authentication Username = Your Email Address
SMTP Authentication Password = Your Password
SMTP Encryption = SSL/TLS
SMTP Authentication = Plain
SMTP Port = 587

Yahoo Users note that you MUST review this: For this to work, in your Yahoo account, change your setting to allow apps that use Less Secure Sign In. Once turned on, Radius will authenticate your Yahoo Account Properly.

Configure SendGrid Email
SMTP Host -- smtp.sendgrid.net
SMTP Authentication Username -- apikey
SMTP Authentication Password -- API Key Code Generated Upon Creation
SMTP Encryption - SSL/TLS
SMTP Authentication - Login
SMTP Port -- 587

Configure HostGator Email
SMTP Host -- Use mail.yourdomain.com
SMTP Authentication Username -- Your Email Address
SMTP Authentication Password -- Your Email Address Password
SMTP Encryption - SSL/TLS
SMTP Authentication - Plain
SMTP Port -- 587

Configure Live[dot]com Email
SMTP Host -- smtp.live.com
SMTP Authentication Username -- Your Email Address
SMTP Authentication Password -- Your Email Address Password
SMTP Encryption - SSL/TLS
SMTP Authentication - Login
SMTP Port -- 587

Configure GoDaddy Email
SMTP Host -- smtpout.secureserver.net
SMTP Authentication Username -- Your Full Email Address
SMTP Authentication Password -- Your Password
SMTP Encryption -- None
SMTP Authentication -- Plain
SMTP Port -- 25

Configure Register.com
Register.com customers:
SMTP Host = mail.websiteaddress (ie mail.site.com or mail.site.org)
SMTP Authentication Username = Your Full Email Address
SMTP Authentication Password = Your Password
SMTP Encryption: SSL/TLS
SMTP Authentication: Plain
SMTP Port: 587

Configure Web.com
Web.com Customers:
SMTP Host = smtp.domainname.com (ie smtp.site.com or smtp.site.org)
SMTP Authentication Username = Your Full Email Address
SMTP Authentication Password = Your Password
SMTP Encryption: SSL/TLS
SMTP Authentication: Plain
SMTP Port: 587

Common Errors

You have a different email address in your Account Profile (Account > My Information > Information) than you're adding to the SMTP Settings. Choose One email address and use that for both.
You are not signed into the proper user profile to set up the SMTP Settings. Make sure you're logged in to the profile of the person who you are setting up if you are an admin.
Not using your full email address but only the part before the @ sign. Use the full email address.
Putting in your Radius password into the place that it says password. Use your email address password.