Complex sentence: During her days as a spy she had missions to recover microfilm and other stolen objects by criminals; she worked for a man trapped in a mailbox costume, codenamed Agent F, and Mitsuki was supposedly the best spy who had ever joined the organization.
Split into simpler sentences: During her days as a spy she had missions to recover microfilm and other stolen objects by criminals. She worked for a man trapped in a mailbox costume, codenamed Agent F, and Mitsuki was supposedly the best spy who had ever joined the organization.