For users that only need a send only email account to send email from our CTS Network read below.
When to Use:
If your domain is not hosted by CTS and your domain hosting company requires you to send email via your ISP’s network. CTS refers to this as hybrid account setup.
To setup your mail box using this method following screens below.

Note: enter size of 0 bytes, and all email will bounce back to sender as this is a send only email box and thus doesn’t need to be managed by the end user. Mail Box will always be empty but user will have capability to send email using the account.

Redirecting account will not matter because the mail box is set to zero bytes and thus email will be bounced to sender. If mail box size is greater than zero, and mail box is not managed box will fill up sooner or later and start bouncing back to senders.