I set up two e-mail accounts, both with activated SMTP.
Unfortunately I couldn’t find a way to set which account should be used as sender.
Reproduce:
CRM → Customer → Choose any customer → E-Mail
You can set a receiver address, cc, bcc but no sender account.
Follow-up question:
Is it possible to set a default sender for specific customers?
Use case: A few of my customers use an e-mail provider that regularly rejects emails from my default smtp server, so I set up a backup account that works.