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When Exchange Online is enabled, this mapping defines the SMTP addresses for target accounts.

When Exchange Online is disabled, this mapping does nothing. If you need to override primary SMTP addresses, map the mail attribute.

When Exchange Online integration is disabled, the SMTP addresses for target accounts are managed by the process which creates the mailbox, and the proxyAddresses mapping has no effect. In this case, if you need to override the account's primary SMTP address, do it with a mail mapping.

Do not attempt to map the mail attribute when Exchange Online is enabled.