How do i send a letter to all customers by e-mail ?

Hello Sales175, If you are wanting to send just a letter to all customers you would have to do this via your own personal email. At present QuickBooks only has the option to batch send transactions/statements etc.

If you want a list of all emails for customers there is a report you can run in the reports section called customer contact list which you could filter to just email and then export to excel and copy all the emails from and then input in your email.