Under https://community.mimecast.com/s/article/email-security-cloud-gateway-configuring-3rd-party-encryption-solutions , mimecast offers a description for the connection of SEPPmail.
After outgoing emails are sent by mimecast, the mimecast SPF should be set in the customer DNS (as of June 01, 2023) to
v=spf1 include:de._netblocks.mimecast.com ~all
which can be looked up under
The forward server to be entered for mimecast at SEPPmail Secure E-Mail Gateway customers usually corresponds to the GINA hostname of the [default] GINA domain as found in the table of the menu GINA Domains.
For seppmail.cloud customers, the forward server is
<customer hostname>-<customer TLD>.mail.seppmail.cloud
For the client email domain
mycompany.tld
the forward server would therefore be
mycompany-tld.mail.seppmail.cloud
