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An SPF record tells receiving mail servers which email server your domain is authorised to send email from. Without an SPF record, there is a greater risk that someone could spoof your domain — making it appear as though an email came from your domain when it actually didn't.
To send via our email server, your SPF record should look like this:
v=spf1 a mx a:smtp2.smartemail.se a:smtp3.smartemail.se a:smtp4.smartemail.se mx:mail.smartemail.se -all
Many email providers now enforce SPF strictly and will mark messages as spam if it is not set up correctly.