timo I came here looking for how to include the email, and the client phone number, on the PDF invoice as well.
It serves two purposes: 1st, it lets the client know that I have their correct A/R information. 2nd, I can immediately see what email address to send the invoice to (I don't always use the built-in email function because I send multiple invoices to the same customer in a single email). Likewise, if the customer asks for a different address to be used, the historical invoices show where they were sent before it was change.
Could you perhaps have a set of fields for the customer contact that's separate from the mailing address, which should not be cluttered with not postal data? Like, Contact Name / Email / Phone number.