If two clients (configured for the same account) open the same S/MIME encrypted mail at the same time, different versions of that mail are saved.
In the mail the following message is shown then: This is the most recent version, but you made changes to another copy. Click here to see the other versions.:

Notes:

• Openpgp mails are not affected, it only happens for S/MIME mails
• The number of versions increases with further concurrent access.

See the mail ted:INBOX/Versions/recent version test (smime) (which has 3 other versions so far)

To reproduce:

1. Open 2 Outlooks with e.g. Ted's account
2. Open an S/MIME encrypted mail in both clients
3. Close mail, select another mail, select the former mail again => Message is shown