Hi Mariano,
We have the same issue. We have set up the email integration with the customer (Register your own domain and redirect outbound messages). Also, on the customer DNS server TXT, DKIM records, have been configured and Verified. In addition to this we added SMTP integration, the customer uses MS Office 365. SendAs permissions were not configured on the mail server.
The email replies and/or outbound emails "On Behalf of Queue" are sent only from the original account, i.e. the account used for SMTP integration configuration. When we turn off SMTP integration, all the emails are sent and all works fine.
Were you able to solve your problem? Is there an additional configuration needed on the mail server?
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Tatjana Knezevic
Star Telecom
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