Outlook attachment encryption

Question

If S/MIME encryption is enabled in an outlook email, are attachments (e.g. an excel file) also protected while in transit between sender and recipient?

Answer 1

S/MIME is not the same as the transport, but the message is digitally signed and encrypted in transit, yes.

The SMTP conversation between servers is also encrypted using TLS in most cases as well ( and can forced between domains if necesssary)

More on S/MIME

https://learn.microsoft.com/en-us/microsoft-365/compliance/email-encryption?view=o365-worldwide

Answer 2

If S/MIME encryption is enabled in an outlook email, are attachments (e.g. an excel file) also protected while in transit between sender and recipient?

As indicated in the Security settings, both message contents and attachments are encrypted.