The Restrict Senders feature is an option to secure to your account by listing certain domains or specific email addresses that are either allowed or not allowed to send through the account.
The feature is located on the 'Settings > Sending Options > Restrict Senders' section of the dashboard.
Restrict Senders can be set to:
'Allow ONLY the senders listed below to send'
'Allow all senders EXCEPT those listed below to send'
If the feature is set to allow only the listed domains/email addresses to send and an unlisted sender attempts to send, or if you list domains/email addresses that are excluded from sending and they attempt to send, then it will fail with a "550 that SMTP sender is not on your allowed list, You can adjust your list on the 'Settings > Sending Options > Restrict Senders' page at app.smtp2go.com." response.