Your migration may complete with failed items. In most cases, failed items will only consist of a) corrupt items and b) large items. Large items may fail to migrate because of connection timeouts. This article explains what connection timeouts mean, and what you can do about them.
My Destination system allows items up to X MB. Why did items which are only Y MB fail to migrate?
The first thing you must understand is that most email systems we migrate from and to enforce many different limits. You could migrate to a system that has a 25MB receive limit, a 50MB send limit, but only a 10MB API create limit. MigrationWiz is subject to the API create limit, not the send or receive limit.
Also, most email systems have a connection timeout. Although the limit may be 25MB, the email system may close any open connection after (for example) 90 seconds, regardless of limits. Unless an item can be transmitted in under 90 seconds, the item will fail to migrate, with a timeout error.
Also, keep in mind that transmitted email data must be encoded, encrypted, encapsulated, etc. For example, Base64 encoding is often used to encode email content. Base64 encoding alone can cause significant inflation, so the limit may be reached faster than expected.
I want all of my items to be migrated. What can I do about this?
First, set realistic goals. For example, if 3 items failed to migrate out of 60,000, the failure ratio is only 0.005% which is extremely low. Second, retry failed items as many times as you want (steps for this). Third, if you have control over the email system, you could increase size and connection limits.
Some items are still failing to migrate no matter how many times I try. What next?
If we are unable to migrate large items because of their size, you will need to handle them manually.
For example, using Outlook, you can create a Search folder which identifies all items larger than X MB across all folders. The method to identify and handle those items may differ depending on your Source and Destination systems. You may also want to enable logging of failed subjects, as explained here.