Exchange Message Size Limited

 Global Setting

Send/Receive Connector Setting

Mailbox Setting

https://docs.microsoft.com/en-us/exchange/mail-flow/message-size-limits?view=exchserver-2019

Recipient limits between authenticated senders and recipients (typically, internal message senders and recipients) are exempt from the organizational message size restrictions. Therefore, you can configure specific senders and recipients to exceed the default message size limits for your organization. For example, you can allow specific mailboxes to send and receive larger messages than the rest of the organization by configuring custom send and receive limits for those mailboxes.

However, this exemption applies only to messages sent between authenticated senders and recipients (typically, internal senders and recipients). For messages sent between anonymous senders and recipients (typically, Internet senders or Internet recipients), the organizational limits apply. For example, suppose your organizational message size limit is 10 MB, but you configured the users in your marketing department to send and receive messages up to 50 MB. These users will be able to exchange large messages with each other, but not with Internet senders and recipients (unauthenticated senders and recipients).