SAM submits emails to be sent to your email server via SMTP protocol, which either accepts the email or returns an error. Usually the errors returned are generic, like "Failure sending email", which means the email server refused the connection due to invalid username/password, port or SSL.
SAM uses the .NET SMTP Client class, which handles all the work of passing the Server-Address, Port, SSL-Enabled, Username, Password, To, From, Subject and Body to the SMTP Server.
Here are some common causes of email sending issues:
Has your Email Password Changed?
The majority of email sending issues are due to users changing the password for their email account and not updating it in SAM. SAM attempts to send an email (via email@example.com email) to users when their email sending settings are not working.
Does Your Email Account Require You Update Your Password?
Some email administrators require that users update their passwords frequently, such as every 60 days, or set new security requirements which your current password no longer meets.
When this occurs, you no longer have access to your account until you update your password. This also prevents SAM from correctly sending emails on your behalf.
Update your password for your email account, then login to SAM and navigate to your Person record and update it there (Advanced Options > Email Sending)
Some errors are temporary, like if the email server is too busy or times-out.
Have You Exceeded Your Sending Limit?
There are also email sending limits, like 500 per day for most Google Apps users, which will cause your all emails to fail after the limit has been reached.
Some email providers, like Google, may require you to login through their website in order to unlock your account if you've had too many failed login attempts.
You Must Enable "Less Secure" Apps (Google Users)
Google Apps has modified their settings for connection with other applications. Your email account may have a setting to prevent less secure apps to access your account. You can change these settings by following the instructions here.