Not all email programs are able to read signed and/or encrypted email. In most cases, you should be able to reply to the sender and let them know that you can't read the message and advise them not to sign or encrypt messages sent to your account.
Using Outlook to Send Signed or Encrypted Messages:
Encrypted or Signed messages can be identified by the icons displayed on the message within the Outlook client.
The icon for a message that is signed and encrypted displays the encryption icon (a lock).
A message that is signed and sent in clear text displays the signature icon (a ribbon).
If you are getting complaints from message recipients that they cannot read messages, review the instructions for setting up your email program and make sure that the Send clear text signed messages when sending signed messages setting is selected.
You will not be able to read an encrypted message on your iPhone or iPad unless you are using iOS 5 or later and have imported your digital certificate.
Gmail or UTMail
If you receive signed message you may receive the body of the message as an attachment called "smime.p7m". You can reply and ask the sender to resend the message without a signature.
Due to the diversity of Android devices it is recommended that the device manufacturer or the email application developer instructions be used for troubleshooting issues reading digitally signed messages. For importing digital certificates on most Android 5.x devices refer to Importing Digital Certificate - Android.
Outlook Web Access (OWA):
Outlook Web Access (OWA) can always read messages that have been digitally signed but will not display encrypted messages. Encrypted messages will display the following in OWA: