Thank you for using Aspose.Email.
It seems that the attachment size by Oulook is limited to 20 MB by default (which can be increased as well) keeping in view different scenarios when a message with such a large attachment can face failure and retried for transmission again and again.
With respect to Aspose.Email, I am able to create a MSG file with attachment size upto 220 MB and then load the MSG file successfully without any error. Can you share with us what kind of problems are you facing while loading a MSG file with large attachment files? Please share your findings with us in detail and if possible, provide us with the sample file that we can use to reproduce the issue at our end.
It may be that the Java heap size be doing the problem. We request you to try increasing the Heap size for Java using options such as -Xms256M -Xmx1024M. Please let us know your feedback then. We will try to assist you as soon as possible.