'QA_XXXXXXX@journal.us3.archivecloud.net' isn't a valid SMTP domain.'
This issue is caused when a full email address is specified where only the domain portion of the email address should be used on the 'Use of Connector' page when setting up the connector.
For example:

To resolve this issue, enter only the domain portion (text to right of @ sign) of the email address provided by support as follows:

Validation during Send Connector configuration fails.