A Merge1 importer fails to send any items to an EWS target: "All mailboxes are full. There is no active mailbox connection."
Problem
An importer sending messages to an EWS target fails with an "All mailboxes are full" error. However, the mailbox is not full.
Error Message
The importer log shows:
2025-05-14 11:01:23,872 [14] INFO Globanet.Ews.Api - Impersonating for 'Target Mailbox'.
2025-05-14 11:01:25,731 [14] WARN MailboxConnectionPool - Failed to create 'Target Mailbox' mailbox connection instance. Error: No mailbox with such guid (ErrorNonExistentMailbox)..
2025-05-14 11:01:25,757 [14] WARN MailboxConnection - Could not establish a connection with the 'Target Mailbox' mailbox.
2025-05-14 11:01:25,789 [14] WARN MailboxConnectionPool - All mailboxes are full.
2025-05-14 11:01:25,804 [14] WARN Merge1.Targets.EwsTarget.EwsTarget - Message #70268508 sending failed: All mailboxes are full. There is no active mailbox connection.. Target permanent error occurred. No need to retry with another connection.
Cause
The mailbox full error is a generic error. Looking in the log, the error message Failed to create 'Target Mailbox' mailbox connection instance. Error: No mailbox with such guid (ErrorNonExistentMailbox).. is seen. The EWS connection uses the mailbox of the Merge1 service account. The service account mailbox needs to be in an Exchange on-premises database and not in Exchange Online.
Solution
Migrate the Merge1 service account mailbox from Exchange Online back to Exchange on premises.
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