Many time End users get an Error message : Unable to Send Email, Invalid ID, while saving the page. This usually means some kind of issue while sending email from your appserver or batchserver using the SMTP Server. The error message is not correct and to get the exact reason, why send email is failing, you need to add/Update the following in your appserver configuration (PSAPPSERV.CFG) or batch server Configuration (PSPRCS.CFG) under SMTP Settings.
1 means enabled, 0 - Disabled. This setting is sometimes not present in PSPRCS.CFG. You have to add it manually opening the file. This setting is dynamic and does not require reboot of the Server, which is very good for troubleshooting in production environments.
Once turned on, it will generate Trace file SMTP.LOG in LOGS Folder under $PS_HOME/tools/appserv/DOMAIN for Appserver and $PS_HOME/tools/appserv/prcs/DOMAIN for Batchserver.
Open SMTP.LOG and fix the issues logged in it. This will make your users to save the page.