.DET File Extension
What is a DET file?
Proprietary email message format used by ACT!, a contact management program; contains a single email message without any attachments.
If an ACT! email contains one or more attachments, each attached file is saved as a separate DET file. The email message includes pointers to each associated DET file, which can be accessed by opening the original message. For instance, if an email contains three attachments, it would be saved as four DET files - one for the message and three for the attachments.
NOTE: The DET format was used in ACT 5.0 to 6.0.2. ACT 6.0.3 and later saves email messages in the .IMA format. Sage ACT! became Swiftpage Act! in 2013.