Web.de / SMTP error code 550
smtp;550 <example@example.com>: Reject due to policy restrictions (header based)
Your message will be rejected by their system if: any of the technical information included in the email doesn't meet the standards, the email violates their guidelines or, it is not plausible, i.e.: when specifying the date. This is applicable to all emails with more than one of the following headers: BCC, CC, Date, From, Sender, Subject, To. In addition, the headers "Date, From, Sender, To" must be syntactically correct.
Documentation
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