How long will a migration take?

This is a tricky one as there are so many things that can affect the duration of a migration. Let's go through them:-

Available bandwidth

We have a huge amount of bandwidth available for you to use with Migration Monster so this is rarely a factor when looking at the speed at which a migration completes. The available bandwidth at the source and destination servers is far more likely to control the speed at which your migration completes. The lower the available bandwidth at the source and destination, the longer the migration will take to run. However, one of the reasons for using Migration Monster is that it allows you to complete the migration over a longer period of time and in the background. Running your migration multiple times allows you to migrate the bulk of the mailbox content well before you plan to switch over to your new email service. Once you have switched your email destination so your email enters your new email server you can run the migration a final time just to update the last few items (which can be very quick). If you're very short of bandwidth at the source and destination you can schedule the migration to only run outside your working hours. It is common to see connection errors within the error logs when low levels of bandwidth are available, however, Migration Monster will attempt to remove such errors at the end of the migration pass.

Number of items to migrate within a mailbox

Migration Monster will migrate up to 500 items per minute in a typical mailbox but this can be affected by the size of the item, what the items are and how large they are. 

Multiple mailboxes

We allow multiple mailboxes to run at the same time which increases the performance of a migration pass.

Mailbox size

This is probably an obvious one, the larger the size of the mailbox the longer the migration will take.

Server capacity

The performance of your migration will also depend on the performance of both the source and destination email servers. If the server is old or overloaded this will also impact the speed of your migration. Poor performing servers may cause connection time-outs which you may see reported in the error logs. At the end of a migration pass Migration Monster will automatically attempt to rectify any errors reported.


Errors can occur for all sorts of reasons like corrupted, oversized items, connections and performance issues. All of these issues will slow the performance of the migration as the system attempts to recover from these errors automatically. If the system is unable to clear the errors automatically it will report them and allow you to attempt to clear them manually by selecting the "retry errors " option from the toolbar.


It is common for many email servers to employ throttling within their configuration. Whilst this assists with the performance across all users of an email server it does reduce the speed of any migration. The migration Monster system detects when throttling is in place and in response will reduce the speed of operation. In some situations, it may be possible to contact the service provider to temporarily disable throttling for the duration of the migration.

If completing a migration to/from Office 365 it is worth raising a ticket request that throttling be reduced. They will normally do this for 90 days on request. If carrying out an Office 365 tenant to tenant migration, remember to raise a ticket for both tenants.