Unfortunately, this is a common issue — spam can appear to come from your own email address even though it doesn't.
To prevent this, you need to add an SPF record to your domain name (via the DNS service). By default, this is already set up on new web hosting and DNS services, but you can control which mail servers are allowed to send email using your domain name. Here's an example if you're using us as your email provider:
"v=spf1 a mx a:smtp2.smartemail.se a:smtp3.smartemail.se a:smtp4.smartemail.se mx:mail.smartemail.se -all"
If you send via other mail servers (booking systems, invoices, or perhaps newsletters through a third party), you can add:
a:annan.mailserver.se before a:smtp2.smartemail.se
Contact us if you need help adding the values.