If you use AWS and try mailing a report to multiple emails it fails with:
Mailing Error Failed to send data [SMTP: Invalid response code received from SMTP server while sending email. This is often caused by a misconfiguration in Outbound Email settings. Please verify the settings at Administer CiviCRM >> Global Settings >> Outbound Email (SMTP). (code: 554, response: Transaction failed: Domain contains illegal character)]
Email Delivery, it does not like the comma in the comma separated format that is supposed to be used for multiple email addresses.