There are different wayt to authenticate emails as being legit. One of the easiest to implement is SPF.
Basically what you need to do is add an SPF record to your domain’s DNS zone. But since most DNS control panels do not have this record, you can simply add it as a TXT record.
The OpenSPF site provides the syntax used to identify the resources on your network or otherwise that are allowed to send email on behalf of your domain. But I always find it easier to use a simple form to generate that record. Below are a few helpful sites that offer such a form/wizard: