Forensic Email Collector (FEC) supports differential acquisitions where the current acquisition can skip messages that were previously acquired in other acquisitions of the same target mailbox.
Differential acquisitions can be activated as shown in the screenshot below:
Differential Identifier Sources
FEC's differential acquisition workflow supports two types of differential message identifier sources:
1. FEC Projects
FEC will scan the specified Differential Acquisition Base Path, including any subfolders, and locate any FEC Projects where the same target mailbox as that of the current acquisition was targeted. The identifiers of any successfully-acquired messages in these previous acquisitions will be used to exclude those messages from the current acquisition.
Note1: When scanning FEC projects inside the Differential Acquisition Base Path, FEC excludes the current project. Therefore, it is okay to create your new project within the Differential Acquisition Base Path. This supports incremental acquisitions where new acquisitions are created inside the same folder periodically.
Note2: FEC looks for successfully acquired items when scanning previous FEC projects. If you create an FEC project (#1) and use it as a differential message identifier source for a second project (#2) without actually running the acquisition in Project #1, Project #2 would find no differential message identifiers as Project #1 would not yet contain any successfully acquired items.
2. Input Lists
FEC will scan the specified Differential Acquisition Base Path, including any subfolders, and locate any input lists named as <target>_DIFF.tsv. For example, if the target mailbox is email@example.com, the corresponding differential input list would be named firstname.lastname@example.org_DIFF.tsv.
The input list should be a tab-delimited text file and should contain the Service ID column as found in FEC's Downloaded Items Log. For IMAP acquisitions, the input list should also contain the Folder column from FEC's Downloaded Items Log. The presence of additional columns is allowed. Therefore, it is possible to use a renamed version of FEC's Downloaded Items Log (in TSV format), or a subset of it, as your input list.
The ability to use an input list instead of a full FEC project allows the following scenarios:
1. Using a list of identifiers from an external acquisition as the basis for a differential acquisition in FEC.
2. Excluding only a subset of a previous FEC acquisition from the current acquisition.
Differential Acquisition Reporting
When differential acquisition is enabled, FEC provides additional information in two areas:
1. Additional statistics are provided in the Acquisition Summary section of the Acquisition Log about how many differential IDs are imported and how many of them overlap with the current acquisition.
2. An additional log file named Diff_Excluded_Items is output inside the Logs folder. This file contains a list of items excluded due to differential acquisition and the corresponding differential identifier source (i.e., FEC project or input list).
Differential Batch Acquisitions
You can combine differential acquisitions with FEC's batch acquisition workflow. The Differential Acquisition Base Path you specify in the main project would be inherited by all the additional projects that are automatically created. However, when launched, each individual project would scan for differential message identifiers relevant for the target mailbox of that project only.
Instead of excluding the items found within the differential identifier sources, FEC can limit the acquisition to those items. To trigger this behavior, select the Use as Inclusion List option as in the screenshot below:
For example, if the differential identifier sources found under the Differential Acquisition Base Path consisted of the Service IDs 1234 and 1235, choosing the Use As Inclusion List option would limit the acquisition to only the two items with the Service IDs 1234 and 1235.