One of the first actions most Exchange Administrators generally take when troubleshooting suspected problems with Exchange Content Indexing will be to rebuild the impacted Mailbox Database's content index files (either manually or by using the ResetSearchIndex.PS1 script found in the \Exchange Server\Scripts directory). Many Exchange administrators choose to proactively rebuild search indexes at various points during the calendar year or as various milestones within a project are met (say in a migration project; as a single example).

Eric Norberg wrote an awesome set of posts regarding this subject which show us much more in detail what to do in each situation… Below we can find them:

We hope you find this post series useful, and better yet, have learned something new along the way!