Exchange Hybrid - On-Prem mailboxes can't send to Online mailboxes
We're now running Exchange Hybrid OnPrem: Exchange 2019 latest patch. On-Prem mailboxes are not able to communicate with mailboxes in Exchange Online in any way. The other way around is working fie though. Mailboxes on-prem: The communication to Exchange Online mailboxes isn't working > Can't send emails to Exchange Online mailboxes, or see free busy of Exchange online mailboxes and can't send calendar invites. Mailboxes in Exchange Online: Receive emails from external sent to: email address removed for privacy reasons - working. Receive emails from external sent to: email address removed for privacy reasons - working. Receive emails form external setn to: email address removed for privacy reasons - NOT working. Receive emails from internal on-prem mailbox - NOT working. No matter to which domain it is sent. Calendars: Users can fully see on-prem calendars and can also send invites. Send emails to external and internal - working. The Exchange on-prem queue shows: DeliveryType: DnsConnectorDeliver Status: Retry NextHopDomain: cadomain.mail.onmicrosoft.com The bounce back emails the on-prem mailbox receives when trying to send to a mailbx that is online: Server at cadomain.mail.onmicrosoft.com (104.47.75.164) returned '400 4.4.7 Message delayed' 3/24/2023 3:38:01 AM - Server at cadomain.mail.onmicrosoft.com (104.47.75.164) returned '450 4.4.316 Connection refused [Message=Socket error code 10061] [LastAttemptedServerName=cadomain.mail.onmicrosoft.com] [LastAttemptedIP=104.47.75.164:25](Socket error code 10061)' This is the on-prem Exchange delivery report: Delivery Report for usernametest2 (usernametest2(at)domain.ca) Submitted3/24/2023 9:25 AM EXCHANGE2019 The message was submitted to on-premExchange.domain.local. Pending 3/24/2023 9:25 AM on-premExchange.domain.local Message was received by on-premExchange.domain.local from on-premExchange.domain.local. 3/24/2023 9:25 AM on-premExchange.domain.local The e-mail address for recipient "usernametest2(at)domain.ca" was updated to the e-mail address "usernametest2(at)cadomain.mail.onmicrosoft.com". The message is in the process of being delivered. Submitted 3/24/2023 9:27 AM on-premExchange.domain.local The message was submitted to on-premExchange.domain.local. Pending 3/24/2023 9:28 AM on-premExchange.domain.local The e-mail address for recipient "usernametest2(at)cadomain.mail.onmicrosoft.com" was updated to the e-mail address "usernametest2(at)domain.ca". The message is in the process of being delivered. 3/24/2023 10:40 AM on-premExchange.domain.local Message delivery is taking longer than expected. There may be system delays. For more information, contact your email admin.