Need Answer
Does mailbox.org offer anti-spoofing options for custom domains? I can find no information about this on the help pages. Other privacy-oriented providers, e.g. protonmail, do offer this (see here: https://protonmail.com/support/knowledge-base/anti-spoofing/).
Mailbox.org should make information about SPF, DKIM and DMARC easy to find.
The same question
You can always just check the DNS Records yourself.
Short anwer: Yes, mailbox.org uses SPF, DKIM and DMARC.
Long answer: Everyone can send Emails in your name anyway. The mailbox.org SMTP servers do not check if you spoof your from address. See this discussion in the german forum: https://userforum.mailbox.org/topic/mailbox-org-smtp-server-stellt-mails-mit-gefakten-absender-zu
Gmail antispam system uses DKIM and DMARC. If I send an email with my custom domain to a Gmail user that will may be redireced to spam
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