In most cases Confluence sends e-mails sothat FROM address is the one configured globally as described here: http://confluence.atlassian.com/display/DOC/Configuring+a+Server+for+Outgoing+Mail, field "From Address".

In case of mentions, however, it tries to use the e-mail address of the currently authenticated user as the FROM address. It still authenticates to the mail server as the globally configured user. This causes problems in case of Exchange mail server when users are not allowed to send e-mails on behalf of others. In this case Exchange throws "550 5.7.1 Client does not have permissions to send as this sender" error.

The workaround is to configure Exchange so that it allowes to send as any sender as described here: http://www.petri.co.il/forums/showthread.php?p=154834.

It would, however, be useful to be able to configure the way Confluence handles FROM addresses in case of mentions. It should be possible to send from the globally configured address in the same way all other mails are sent.