Regular JIRA notifications all come form the same smtp address. The share/mention tries to use the email address of the user sending the share/mention. This can cause problems in exchange mail servers when users are not allowed to send emails from other accounts. This results in an error.

at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859)
        at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
        at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
        at java.lang.Thread.run(Thread.java:662)
Caused by: com.sun.mail.smtp.SMTPSendFailedException: 550 5.7.1 Client does not have permissions to send as this sender

        at com.sun.mail.smtp.SMTPTransport.issueSendCommand(SMTPTransport.java:2057)
        at com.sun.mail.smtp.SMTPTransport.finishData(SMTPTransport.java:1862)
        at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1100)
        at com.atlassian.mail.server.impl.SMTPMailServerImpl.send(SMTPMailServerImpl.java:156)
        ... 128 more

You can see the difference between the following DEBUG logs

Regular notification

2012-02-23 14:43:19,593 Sending mailitem To='email@email.com' Subject='(CPCK-108) cupcake' From='null' FromName='ADMIN (JIRA)' Cc='null' Bcc='null' ReplyTo='null' InReplyTo='null' MimeType='text/html' Encoding='UTF-8' Multipart='null' MessageId='null' DEBUG admin 883x167x1 yxacmf 172.28.15.218 /secure/admin/MailQueueAdmin.jspa [com.atlassian.mail.outgoing] DEBUG SMTP: Found extension "SIZE", arg "35882577"

Mention/Share

2012-02-23 14:43:16,181 Sending mailitem To='fake@user.com' Subject='' From='email@fakedomain.com' FromName='ADMIN (JIRA)' Cc='null' Bcc='null' ReplyTo='null' InReplyTo='null' MimeType='text/html' Encoding='UTF-8' Multipart='null' MessageId='null' DEBUG admin 883x167x1 yxacmf 172.28.15.218 /secure/admin/MailQueueAdmin.jspa [com.atlassian.mail.outgoing] YXRsYXNzaWFuaXZhbkBnbWFpbC5jb20=

Currently, I am not aware of any workarounds.