Email test says email address is invalid although it is used to send emails

When in 3rd party integrations I do a test of an email account by clicking the email icon I get an error message on any email address of our organization I enter as recipient which ends in .training (the ending of our domain). etc. stating the email address is invalid.
This is confusing as email addresses using that ending are successfully used to send emails in the automation rule.
I have tested 2 email accounts (one with a .training email and another with a different .de email- same result.