There are several preference setting related to this, depending on what version you're using. For example,
Acrobat 9: Edit > Preferences > Security > Digital Signatures > Verify signatures when the document is opened
Acrobat 11: Edit > Preferences > Signatures > Verify signatures when the document is opened
To fully validate, they will need to add your certificate to their list of trusted certificates or you can set up the Windows Integration feature under Preferences.
For windows integration to work the signing certificate or one of its parents need to be trusted on the recipient's machine. ALternately it needs to be added to the Trusted Identities on the recipients' machines. You can use Export/Import Security Settings feature in Acrobat XI Pro (Edit->Preferences->Security) to propagate trust within your organization.
Thanks George, that was helpful.
Hey isakten, many thanks for your help...really appreciate it.