Her står det hvordan man gør ifølge google:
https://support.google.com/a/answer/174124?hl=en

Her kan man læse om hvordan man sætter det op i simply.com
https://www.simply.com/dk/support/faq/2/136/

Hvis jeg skal sende plakaten mail gennem google mail servere, ser det ud til at jeg skal gøre som der står her: https://support.google.com/googlecloud/answer/10756302?hl=en
Ordforklaringer:
SPF = Sender Policy Framework Sender Policy Framework (SPF) is an email authentication protocol that domain owners use to specify the email servers they send email from, making it harder for fraudsters to spoof sender information
TXT = A TXT record (short for text record) is a type of resource record in the Domain name system (DNS) used to provide the ability to associate arbitrary text with a host or other name, such as human readable information about a server, network, data center, or other accounting information.