I have recently setup S/MIME in Outlook for my company's employees to send encrypted and digitally signed emails to one another. Is it possible to send encrypted emails to other email platforms from Outlook? If not, then what is the safest way to transmit sensitive information over email?
Encrypted E-mail requires a bit of work by somebody. You've done the work for your users. Now the question is what's going on at the other end. The person on the other end will need to create (or have created for them) an SMIME certificate and private key. Then the folks at each end will want to exchange SMIME certificates.
Many mail clients can be configured to send out the user's SMIME cert as an attachment so that their correspondents will have a copy of it after receiving a single unencrypted message. This allows the folks at the other end to use that cert to send an encrypted message to the user.
The tough part is getting folks on the far end to generate their own keys & cert and getting that cert into the hands of your users so that they can send encrypted messages outbound. Sorry to say, but getting that cooperation from the far end may not be easy...
Yes, you can send S/MIME encrypted messages to other mail systems once you've set it up at your end. But the people you're emailing also need to set up S/MIME at their end and sadly S/MIME is not universally supported. Although Outlook and Thunderbird support it my experience is most mail clients don't, especially on Android. And it's a similar story with PGP too. As for web-based email you're completely stuffed.