Google/Gmail Troubleshooting mail not delivered

Mails are not delivered:
https://support.google.com/mail/troubleshooter/2935079

In Gmail search for in:anywhere rfc822msgid:[mailid]

About spam:
https://support.google.com/mail/answer/1366858

General Guidlines

• Use separate sender addresses foreach category.
  • Examples: newsletter@example.com, booking@example.com, payment@example.com, registration@example.com
• Add auto submitted header
  • https://www.iana.org/assignments/auto-submitted-keywords/auto-submitted-keywords.xhtml

  • Auto-Submitted: auto-generated

Bulk Senders Guidelines

https://support.google.com/mail/answer/81126?hl=de

• Add unsubscribe header
  • https://www.ietf.org/rfc/rfc2369.txt

  • List-Unsubscribe: <http://www.host.com/list.cgi?cmd=unsub&lst=list>, <mailto:list-request@host.com?subject=unsubscribe>

• Add bulk header
  

  • Precedence: bulk 

Certifed Senders Alliance (CSA)