Depending on organizational requirements, a migration project may call for a large number of users to move from the old to the new environment in a very short period of time, such as overnight or over a weekend. Under these circumstances, activities such as email software deployment and end-user training are often performed in advance of the migration date, but mail migration is usually left until the very last moment. Thus, mail migration for a large number of users is executed in a limited amount of time.

This article introduces and explores the concept of incremental mail migration, which solves the problem of latency in mail migration and ensures predictability and seamless mail migrations. My company, Binary Tree, makes a product called CMT (Common Migration Tool) that supports incremental mail migration, so we've been playing with this problem for quite some time. That said, this article is about technology and not about my product.

What's the problem?
Initially, the decision to perform the entire migration in a very limited time period may appear logical, but it could cause the downfall of an entire migration project. The migration of data from one system to another is a complex and a time-consuming task depending on a variety of factors such as reading, translating, and writing data. Network bandwidth, migration workstation capacity, availability and response of mail servers, and, finally, sheer volume of migrated mail dramatically impact the duration of mail migration so that it often takes significantly more than the allotted time.

Imagine a situation where end-users, who've been trained in using the new email environment with a promise that all their old email will follow them to the new environment on a certain day, find that instead only a subset of email data is in the new environment, and the remainder is trickling in gradually. Worse yet, some users find all the old mail in the new environment while others find none, feeling singled out and frustrated at the entire migration process. Such a situation represents an utter support nightmare, and yet they're quite realistic, and even quite likely.