Check your SPF
Check SPF record of your domain!
What is SPF?
The Sender Policy Framework (SPF) is an authentication protocol that theoretically protects against domain spoofing.
SPF introduces additional security checks that enable a mail server to determine whether a message originated from the domain, or whether someone is using the domain to mask their true identity.
SPF records are typically defined using the TXT record type. There’s also an SPF record type, but it’s deprecated, and you should always have at least the TXT record definition present, even if you use the SPF type.
SPF records are defined as a single string of text. Here’s an example record:
v=spf1 a mx ip4:126.96.36.199 include:_spf.emailclient.com ~all