Notification emails sometimes works sometimes don't

[Szymon Ptak]

We have configured emails that are resend from test@domain.com to jira's default email handler.

Scenario 1 : 

Customer sending email -> jira makes issue -> automation rule sends notifications : ALL OK

Scenario 2 :

Customer sending email -> jira makes issue -> automation rule fails.

Error : 

Missing final '@domain' (Service: AmazonSimpleEmailService; Status Code: 400; Error Code: InvalidParameterValue; Request ID: 565dd9ad-0d23-47a6-9655-f1cadced99a5; Proxy: null)

It's like im doing the same thing but sometimes there is an error. What could be the source of that?

All of the other notification emails work flawlessly, so Im guessing there's something wrong with forwarding emails to jira from our test@domain ?