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To add a personal domain do the following:

• Go to Settings > Administration > Personal Emails 
• In the section under "Add New Personal Email" enter in the @ symbol followed by the domain name for the personal domain name you want to add
• Click the Add New Personal Email > App Setup > Personal Domains
• Click the New button

To edit or delete a personal domain do the following:

• Go to Settings > Administration > Personal Emails In the table under "Personal Emails" click App Setup > Personal Domains
• Click Edit or Delete under the Actions column next to the respective domain

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Image AddedBlocking Form Submissions from Personal Email Domains

See the directions here for more information.