If your SMTP connection was working fine, but now you suspect that your clients are not receiving your emails, the chances are you have changed the password for your email account without updating it in SMTP.
There are two things you can do to verify this;
1. Go to ADMIN > SETTINGS > SMTP Email Settings and click "Test & Update". Typically an error message such as the one shown below will verify that your password is no longer valid in your email account.
2. Take a look in the SMTP STATUS report.
Navigation - REPORTS > SMTP STATUS
Your SMTP STATUS report show all emails that have failed to be sent successfully along with a reason for the failure.
A failure notice such as this one
indicates that your email password is no longer valid and needs updating in
ADMIN > SETTINGS > SMTP Email Settings.