To set up an account that receives incoming mail from your ISP, but sends outgoing mail through Pobox, please follow these instructions.
The next screen will provide you with a list of options. Select the last option, "Add Other Mail Account" and click on "Continue":
****If you do not see this next screen during setup, please skip.
If you do see this screen with the error message "unable to verify account name or password" Please enter:
Email Address: Your Pobox email address
Incoming Mail Server: Incoming server for your Forwarding address
Outgoing Mail Server: SMTP.pobox.com
Click "Sign In" and you will be redirected to the mail account settings page (skip the next 2 steps if you did not receive the below screenshot)
Finish by clicking on create.
After hitting Create, go bace to the 'Account Information' page and select 'Edit SMTP Server List...'
On the following page, you will need to update your the SMTP settings for your email program. What you'll need to do is switch your port to 587, and change your authentication to Password. In the User Name and Password fields below, enter your Pobox username and Pobox password (app-specific password is you have Two-step verification enabled). Finish the setup process by clicking OK.