Replace values in one column based on 3 criteria

Hi,

Complete novice when it comes to Custom Columns in Query Editor.

I have two columns, Primary email and Secondary email. I have a few condtions that I need met for my new column.

1. If Primary email is blank, return the value in Secondary email, only if Secondary email contains '@emailaddress'
2. If Primary email contains '@snailmailaddress', replace with value in Secondary email, provided it contains '@emailaddress'
3. If Primary email is blank, and so is secondary, then return blank

Any guidance on this would be appreciated.