Message size limits There are basically three places where you can configure default message size limits on Exchange:

• Organization transport settings
• Send/receive connector settings
• User mailbox settings.

To check your server’s current limit you can open and access them via Exchange Management Console (EMC), however you can use PowerShell as a method for checking message limits as well. Run the following code in the Exchange Management Shell:

get-transportconfig | ft maxsendsize, maxreceivesize 
get-receiveconnector | ft name, maxmessagesize 
get-sendconnector | ft name, maxmessagesize 
get-mailbox Administrator |ft Name, Maxsendsize, maxreceivesize

The last line of the script (Get-Mailbox) returns information only about the 'Administrator' user's mailbox. Replace 'Administrator' with the user name that you want to check. Or, you can omit the username Administrator (and any other username), and the result will be a list by username of all mailboxes that are present on the Exchange along with their respective mailbox size limits (those size limits likely being 'unlimited' because limits per mailbox were not ever set or configured - which is fine too).

get-mailbox |ft Name, Maxsendsize, maxreceivesize

After you get-transportconfig and get-receiveconnector and get-sendconnector, you can use PS Script in the Exchange Management Shell to change the existing message maximum size limits. The following example shows how to change the size of messages accepted by the transport service to 100 MB.

Set-TransportConfig -MaxSendSize 100MB -MaxReceiveSize 100MB

The easiest way (the least amount of typing) to set the maximum message size limits for the exchange send and receive connectors is to log into the Exchange Admin Center as a user with appropriate administrative privileges.

https://yourmailhostname.yourdomain.tld/ecp/

and do the following:

• Select 'mail Flow' from the left menu.
• Select 'receive connectors' from the horizontal submenu
• Select and separately edit each receive connector in the list to change the Maximum receive message size (MB), and click the “Save” button.
• Select 'send connectors' from the horizontal submenu
• Select and edit the listed send connector to change the Maximum send message size (MB), and click the 'Save“ button.

To confirm your changes, open the Exchange Management Shell [PS] and enter the following:

get-transportconfig | ft maxsendsize, maxreceivesize 
get-receiveconnector | ft name, maxmessagesize 
get-sendconnector | ft name, maxmessagesize 

• In EAC → select Recipients → Select Mailboxes
• Select a Mailbox user → click Edit
• On the mailbox properties page → click Mailbox Features.
• Under Message Size Restrictions → click View details to view and change the following message size limits:
• Sent messages: To set a maximum size for messages sent by this user, select the Maximum message size (KB) check box and type a value in the box. The message size must be between 0 and 2,097,151 KB. If the user sends a message larger than the specified size, the message will be returned to the user with a descriptive error message.
• Received messages: To set a maximum size for messages received by this user, select the Maximum message size (KB) check box and type a value in the box. The message size must be between 0 and 2,097,151 KB. If the user receives a message larger than the specified size, the message will be returned to the sender with a descriptive error message.
• Click OK, and then click Save to save your changes.

Open Exchange Management Console Power Shell [PS]. Here is an example that sets the maximum size for sent messages to 50 MB and the maximum size for received messages to 36 MB for the mailbox of James Geek.

Set-Mailbox -Identity "James Geek" -MaxSendSize 50mb -MaxReceiveSize 36mb

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

https://www.codetwo.com/admins-blog/exchange-attachment-size-limit-find-change/

Setting Message Size Limits Per Mailbox:

https://docs.microsoft.com/en-us/exchange/recipients/user-mailboxes/mailbox-message-size-limits?view=exchserver-2019