Email Retention in Exchange 2010

I am looking for a formula on how to calculate email retention policy in Exchange 2010.  We are currently using an external archiving solution and we can define retention policy within this solution.

Our current database size is 2.5TB
Total mailboxes is 3000 users

Therefore average mailbox in Exchange is about 833MB

If I were to apply a retention policy of 180 days or 90 days.   What will be the new total storage size.
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akoladeConnect With a Mentor Author Commented:
Thanks for this.  WIll givce this a try
Bruno PACIIT ConsultantCommented:

The new total storage size, if you're talking about file size, will be 2.5 TB... Because as always since Exchange exists, the database files never shrink by themselves. In fact the database file will have empty blocks that will be reused by Exchange but the file size will nver decrease.

Ok, so now, about a formula to estimate the data size (I use "data size" term and not "database size" for the reason I explained above) you need these values :

- The average message size: You can calculate this very precisely using Get-MailboxStatistics cmdlet (ex : Get-MailboxDatabase “DB01” | Get-MailboxStatistics | FT TotalItemSize, ItemCount). You can export the needed values in a CSV file with Export-CSV and use Excel to calculate the message size average.
- The number of mailboxes in each database: very easy to obtain
- The average number of message received per day (coming from internal of external): You can use result of the Message Tracking Log for that with the cmdlet Get-MessageTrackingLog.

The total data size in the whole platform is equal to:
Total data size = (average number of message received per day) x (average message size) x 180 (for 180 days of retention)

The data size in each database (the database payload) can be obtained like this:
Data size in database = ((Total data size) / (Total number of mailboxes)) * (Number of mailboxes in the database)

What must be added is the deleted messages retention.

With all that you should estimate quiet precisely your "real" data size per database, but again this as no link with the database file size for reasons alreayd explained. What is sure is that the files can not be smaller than the data.

Have a good day
akoladeAuthor Commented:
Only solution I can find on online t that is sraight forward
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