Confluence doesn't send emails

After update to release 5.1 Confluence seems not to be able to send emails (notifies, shares, etc.) anymore.
Here is the error I can see in the log:

2013-06-21 15:36:15,352 INFO [scheduler_Worker-6] [atlassian.crowd.directory.DbCachingRemoteDirectory] synchroniseCache FULL synchronisation complete for directory [ 38830084 ] in [ 1625ms ]
2013-06-21 15:36:16,836 INFO [read-only-transaction:thread-1] [atlassian.confluence.user.DefaultUserAccessor] getUserNamesWithConfluenceAccess Could not find group configured with USE permission: null
2013-06-21 15:36:23,993 WARN [http-8443-5] [atlassian.upm.pac.PacClientImpl] unknown Error when querying application info from MPAC: com.atlassian.marketplace.client.MpacException: org.apache.http.conn.ConnectTimeoutException: Connect to marketplace.atlassian.com:443 timed out
– referer: https://172.16.20.31/start/display/MP/8.+Cluster | url: /start/display/MP/Merchandise+Platform+Home | userName: Alessandro_tc
2013-06-21 15:36:24,524 ERROR [http-8443-1] [crowd.manager.application.ApplicationServiceGeneric] authenticateUser Directory 'Upgraded atlassian-user LDAP (ldapRepository1)' is not functional during authentication of 'barbaro'. Skipped.
2013-06-21 15:38:00,305 ERROR [scheduler_Worker-1] [atlassian.core.task.AbstractErrorQueuedTaskQueue] handleException com.atlassian.mail.MailException: javax.mail.SendFailedException: Invalid Addresses;
nested exception is:
com.sun.mail.smtp.SMTPAddressFailedException: 550 No such domain at this location

com.atlassian.mail.MailException: javax.mail.SendFailedException: Invalid Addresses;
nested exception is:
com.sun.mail.smtp.SMTPAddressFailedException: 550 No such domain at this location

at com.atlassian.mail.server.impl.SMTPMailServerImpl.send(SMTPMailServerImpl.java:193)
at com.atlassian.confluence.jmx.JmxSMTPMailServer.send(JmxSMTPMailServer.java:80)
at com.atlassian.mail.queue.SingleMailQueueItem.send(SingleMailQueueItem.java:38)
at com.atlassian.mail.queue.AbstractMailQueueItem.execute(AbstractMailQueueItem.java:57)
at com.atlassian.core.task.AbstractErrorQueuedTaskQueue$TaskDecorator.execute(AbstractErrorQueuedTaskQueue.java:107)
at com.atlassian.core.task.AbstractTaskQueue.flush(AbstractTaskQueue.java:45)
at com.atlassian.core.task.AbstractErrorQueuedTaskQueue.flush(AbstractErrorQueuedTaskQueue.java:37)
at com.atlassian.quartz.jobs.TaskQueueFlushJob.doExecute(TaskQueueFlushJob.java:32)
at com.atlassian.quartz.jobs.AbstractJob.executeInternal(AbstractJob.java:88)
at org.springframework.scheduling.quartz.QuartzJobBean.execute(QuartzJobBean.java:86)
at com.atlassian.confluence.setup.quartz.DelegatingClusterAwareQuartzJobBean.executeJob(DelegatingClusterAwareQuartzJobBean.java:16)
at com.atlassian.confluence.setup.quartz.AbstractClusterAwareQuartzJobBean.surroundJobExecutionWithLogging(AbstractClusterAwareQuartzJobBean.java:64)
at com.atlassian.confluence.setup.quartz.AbstractClusterAwareQuartzJobBean.executeInternal(AbstractClusterAwareQuartzJobBean.java:46)
at org.springframework.scheduling.quartz.QuartzJobBean.execute(QuartzJobBean.java:86)
at org.quartz.core.JobRunShell.run(JobRunShell.java:199)
at com.atlassian.confluence.schedule.quartz.ConfluenceQuartzThreadPool$1.run(ConfluenceQuartzThreadPool.java:20)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:549)
Caused by: javax.mail.SendFailedException: Invalid Addresses;
nested exception is:
com.sun.mail.smtp.SMTPAddressFailedException: 550 No such domain at this location

at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:1873)
at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1120)
at com.atlassian.mail.server.impl.SMTPMailServerImpl.send(SMTPMailServerImpl.java:166)
... 16 more
Caused by: com.sun.mail.smtp.SMTPAddressFailedException: 550 No such domain at this location

at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:1724)
... 18 more