Inter-Mailer Cleansing requires that the Mailer upload their list (Plain Text or MD5) to our system to cleanse against the Suppression File. During this process their list will also be cleansed against the other Mailer's lists within the Campaign.
The benefit to this is that it will ensure that duplicate email recipients do not receive the same email multiple times.
For example - If you assign 5 Mailers to a Campaign, the first Mailer to upload their list will have their file scrubbed against the Suppression List only. The second Mailer will have their file scrubbed against the Suppression File AND the first Mailer's list. The third will have their scrubbed against the first two Mailers' lists as well as the Suppression File, and so on.
This does require that OPTIZMO store the Mailer's file until all cleanses are complete. As with any one-off cleanse, the data is purged in full after the job is complete, and we will not store or use the Mailers file(s) for any other reason.