If you view the transaction logs on your Exchange server, you may find that the log files grow rapidly when a user is logged into multiple Outlook sessions.
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This problem occurs if you have the group policy enabled for the Warn me about suspicious domain names in e-mail addresses setting in Outlook and a user accesses their mailbox from multiple Outlook clients configured in Online mode.
When this policy is enabled, you will find the following data in the Windows Registry.
Note, x.0 in the above registry path represents the version of Outlook you are using (15.0 = Outlook 2013, 14.0 = Outlook 2010, and 12.0 = Outlook 2007).
To resolve this problem, change the Hide warnings about suspicious domain names in e-mail addresses group policy setting to Not Configured. The following figure shows the path to the policy setting in the group policy editor.
If you need to keep this policy setting enabled, you have two other possible resolutions.
Use only Cached mode profiles to access your Exchange mailbox.
If you are using Online mode profiles, access your Exchange mailbox using only one client at a time.
By default, the Warn me about suspicious domain names in e-mail addresses option is enabled in the Junk E-mail Options dialog box in Outlook. The default configuration is shown in the following figure.
This setting is used to provide a warning if the domain name in an e-mail address for a message you receive is considered suspicious by the Junk E-mail filter in Outlook.
If you enable the Hide warnings about suspicious domain names in e-mail addresses group policy setting, the Warn me about suspicious domain names in e-mail addresses option is disabled in the Junk E-mail Options dialog box.