Recently faced mail flow issue that users from the databases DB9 to DB12 have received emails without any issue but sent emails have been queued in their Outbox (Outlook Cached Mode users) or in their Sent Items folder but recipients didn’t receive it (Outlook Online Mode users) or in their Draft (OWA users).
We have 5 Mailbox Servers (EXCHMBX01 to EXCHMBX05) in a DAG (DAG01), EXCHMBX01 to EXCHMBX03 are in production site with 4 Databases per mailbox server for the total of 12 Databases and EXCHMBX04 & EXCHMBX05 are in DR site with the database ActivationPreference 3. We have total of 3 HUB/CAS Servers (EXCHHC01 to EXCHHC03), 2 HUB/CAS in the production site and 1 HUB/CAS in DR site.
We found that one of the mailbox server (EXCHMBX03) & HUB/CAS server (EXCHHC03) has faced high performance issue due to auto-update happened in Antivirus software then antivirus scan was executed automatically in the background and even we are unable to RDP the server. At the same time, only auto-update of Antivirus task was executed without system scan in rest the mailbox servers (EXCHMBX01 & EXCHMBX02) & HUB/CAS servers (EXCHHC01 & 02), so we didn’t face much performance issue in EXCHMBX01 & 02 and EXCHHC01 & 02
As EXCHMBX03 was unable to work properly, mailbox databases (DB9 to DB12) in that mailbox server automatically tried to switch over to another servers. Since there was more load in rest of the mailbox servers (EXCHMBX01 & EXCHMBX02) in the production site, DB9 to DB12 mounted in EXCHMBX05.
As mentioned earlier that HUB server (EXCHHC03) also has high performance issue, so all mailbox servers should notify the available HUB servers (EXCHHC01 & 02) that new mail is available then HUB server will picks the mail and place it in Submission Queue in HUB server. All the emails from EXCHMBX01 & 02 were successfully submit to either EXCHHC01 or EXCHHC02 but emails from EXCHMBX05 emails are unable to submit to anyone these HUB servers.
While troubleshooting we found that TransportSubmissionServerOverride was set in the mailbox server EXCHMBX05 to submit the emails only to EXCHHC03. So we have removed the values and marked it as Not Set then emails were submitted to EXCHHC01 & 02. To verify this option, please refer the screen captures of a mailbox server from ADSI Edit:
Also we have PowerShell command to verify as well as to set the override value:
Get-MailboxServer | ft name,SubmissionServerOverrideList
Set-MailboxServer “EXCHMBX05” –SubmissionServerOverrideList “EXCHHC03”
Set-MailboxServer <Mailbox ServerName> -SubmissionServerOverrideList $null
Note: The PowerShell command above will not check whether we are updating a valid HUB Server to SubmissionServerOverrideList.