Maybe Google adds feature to send scheduled e-mails in Gmail:
https://www.androidpolice.com/2018/07/29/gmail-v8-7-15-prepares-add-email-scheduling-apk-teardown/
If then I switch back from Inbox to Gmail.
BTW: I’ve got a notice that Google pushes Gmail and let Inbox die.
Update 01.12.2018: Google stops Inbox end of march 2019.