Am unable to provide domain name in User name filed for Email listener service, when I try to provide domain name it gives error as "invalid character in username".
But to connect the mail service I have to provide user details in format (domain\user) which am unable to provide in Email listener service. With browser am able to connect without any issues.
I've just tested this in version 9.3 of the Gateway and I get the same error. From reviewing the case that was logged for this, the expectation is that the value can not have any special characters outside of _,-, or @. If this is an Active Directory protected email server, you may look to use the userPrincipalName such as email@example.com instead of sample\usera.