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If customer signed up by email:
Ask customer to check the spam folders, the email should be there if correct email address was used
If customer signed up by Google:
If possible, ask customer to log in via Google - We can not change customers google password
If customer wants to log in via email-password an email login option has to be created to customer
Ask customer to use signup flow:
Customer has to enter the same email and set password - This will create email login option for the same customer account
Customer receives an email to verify the address
After that customer can login via email or and via google both
Or signup for the customer:
Enter the same email and a temp password
Customer receives an email to verify the address - Customer does not have to verify account in this step
Now choose to reset password
Customer receives email to change password, when customer created new password, the account will be verified and customer can log in via email.