The following DNS SPF record should be configured for domains sending email via the STRATO email platform:
v=spf1 redirect=_spf.strato.com
This resolves an issue whereby STRATO customers are unable to email gmail.com users.
Emails sent to gmail.com are rejected with the following message:
550 5.0.0 host gmail-smtp-in-l.google.com said 5.7.26 This mail is unauthenticated, which poses a security risk to the sender and Gmail users, and has been blocked. The sender must authenticate with at least one of SPF or DKIM. For this message DKIM checks did not pass and SPF check for [Domain] did not pass with IP [IP Address].