Cannot view digitally signed emails in gmail and yahoo

I have configured my outlook to send digitally signed emails. It works fine when sending to other outlook users. When the recipient  is using gmail or yahoo for example, the body of the email is sent as an attachment. This attachment cannot be opened within the email client and the only option is to save. Is there a way for gmail and yahoo recipients to be able to easily view the content of signed emails?
CBBNet AdminAsked:
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David FavorLinux/LXD/WordPress/Hosting SavantCommented:
The easiest way to do what you're describing is to enable IMAP access to your Gmail + Yahoo accounts.

Then use a mail client like Thunderbird which allows easily adding extensions to auto-decrypt encrypted messages.

If you use GSuite (paid Gmail) + stick with S/MIME encryption, then https://developers.google.com/gmail/api/guides/smime_certs provides a good starting point for doing auto encryption/decryption of messages in + out of Gmail.
CBBNet AdminAuthor Commented:
Thanks for the info. Excuse me, i am not too knowledgeable of how this works, but from what i am understanding from your response is that the issue is due to the decryption of an email that outlook encrypted. (Please correct me if i am wrong). If this is the case, is it possible to sign the email without encryption?
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