This is a tough one because Shopify requires an email to successfully import, and as you point out, hardcoding a fake email would cause all the customers to have the same email.
So the fastest way to make this work would be to open your .csv sheet in Excel and create three new columns: ‘prefix’, ‘suffix’, and ‘combined’. In the prefix column, type into the first box something like ‘customer1’ and in the next box type ‘customer2’. Then you should be able to select these two boxes and drag this box down the length of the sheet so that there is one entry for each row that needs an email.
Next, in the suffix column, type in ‘@noemail.com’. Drag this value down the column so that again, there is an entry for every row that needs an email. Now in column three, enter the formula: =(first cell in prefixcolumn)+(first cell in suffix column). This should give you something like ‘customer1@noemail.com’. Then again, select the cell and drag it down so that it creates an email for every row. Now you’ll want to select this entire row, copy it, and then select your actual email column. Go to the top bar in excel and click the small arrow to the right of the ‘Paste’ button which should give you a small menu. Select ‘Paste Values’.
Now you can delete the three columns that you added and will have a unique email for every customer.
Let us know if this doesn't make sense and we'll be happy to help explain it further!
