When preparing an Exchange system for migration, you should ensure that message size limits are high enough. Exchange is a complex product different types of message size limits: some for sending emails, some for receiving emails, some for Exchange Web Services, some for maximum item size, some for maximum attachment size, etc.
From our perspective, API limits is what matters, and we provide specific guidance to configure relevant Exchange parameters. Now, we recently helped a customer having items with large attachments migrate to Exchange. Some items with a size of 20 MB failed to migrate, even though all Exchange limits were set to 35 MB or higher. Here are some explanations you may find useful.
There are two things you should keep in mind. The first, is that you need to use correct math. For example, setting a limit of 35,000,000 will only result in a 33.3 MBs limit. This is because one KB is actually 1024 bytes, and one MB is 1024x1024 bytes - not 1,000,000 bytes. The second is that packet size limits should be higher. Consider the case of an IIS server accepting packets smaller than 35 MBs and a message with a 20 MB attachment.
The attachment will be encoded in MIME format, which increases size by a factor of 1.37 due to Base64 encoding. If this data is migrated using Exchange Web Services, SOAP encoding will again increase size by another factor of 1.37. Which means IIS will receive a request of size 20 MB x 1.37 x 1.37 = 37 MBs which is more than the 35 MB limit. As a result, the message will be rejected.