UM can handle messages of MBs in size. The default max message size in recent UM versions is 20MB (in earlier versions it ws 1MB).
The difference is purely a config setting and it is simply designed to protect the server from large messages. So if you have large messages and UM has enough memory assigned, you can safely increase that setting.
But you should also determine whether a messaging product is the right way to deliver data of such a size. Such files may be more reliably transferred using a Managed File Transfer product, where you only transfer associated metadata through messaging.
This doesn’t only apply to UM, but would be relevant for most messaging products.
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