If you had SMTP Pro working and without any notice it stopped to send email check if you have even Ebizmarts Mailchimp extension.

If you have both extensions try to go in System -> Configuration -> SMTP Pro -> Logging and Debugging and here try to Run a Self Test.

If you have a failure notice with this error Ebizmarts_Mandrill_Model_Email_Template here you have a quick (dirty) fix.

with FTP go to app/code/community/Ebizmarts/Mandrill and open config.xml and remove this block

<core>
    <rewrite>
        <email_template>Ebizmarts_Mailchimp_Model_Email_Template</email_template>
    </rewrite>
</core>

Save and reupload the config.xml

Clear Magento cache and try again to do the SMTP Pro Self Test, the problem should be fixed and it will start again to send email.