Hi @Rainer_Goergen, the deliverability of emails is determined by your host or SMTP service you are using. That it comes and goes suggests something is changing in those components.
A big factor in deliverability is the level of trust from the mail server - trust that the emails really are coming from the owner of your domain or email address. As before, I recommend using mail-tester.com to see if that sheds any light on the problem:
Contact form problem - #16 by Ben
Some of the steps in our documentation for Bluehost may also be relevant to improving deliverability:
https://backlight.me/docs/bluehost-issues#sending-email