the main point that fix my problem which occured after moving to new farm was the following

3. Is your Exchange server allows SharePoint to Send Mails? In Exchange Server, If anonymous Relay settings is not enabled, then you must add your WFEs in the relay list.? M**ake sure your ALL Sharepoint WFE'**s IPs are added in allowed Relays or Receive connectors in Exchange Server. If you use multiple domains, add all of them. http://technet.microsoft.com/en-us/library/cc288949.aspx Watch the 'DROP' folder in the mailroot of the SMTP server.

Read more: http://www.sharepointdiary.com/2012/02/sharepoint-alerts-not-working-troubleshooting-checklist.html#ixzz3C3jbADd3

adding all the servers (except database servers) in the Exchange relay list fix the problem

the main point that fix my problem which occured after moving to new farm was the following

3. Is your Exchange server allows SharePoint to Send Mails? In Exchange Server, If anonymous Relay settings is not enabled, then you must add your WFEs in the relay list.? Make sure your ALL Sharepoint WFE's IPs are added in allowed Relays or Receive connectors in Exchange Server. If you use multiple domains, add all of them. http://technet.microsoft.com/en-us/library/cc288949.aspx Watch the 'DROP' folder in the mailroot of the SMTP server.

Read more: http://www.sharepointdiary.com/2012/02/sharepoint-alerts-not-working-troubleshooting-checklist.html#ixzz3C3jbADd3

adding all the servers (except database servers) in the Exchange relay list fix the problem