Exchange 2019 refusing attachments within permitted size

I have external users (different mail host) trying to send messages to internal users on our Exchange 2019 server. The attachments are large (7MB-10MB before Base64 encoding), and the senders are receiving the following error after sending:

Remote Server returned '552 5.3.4 Message size exceeds fixed limit'

I checked my settings as show below, and my exchange server should be accepting messages up to 25MB. Can someone explain how to diagnose/resolve this?

As well, does this look like an exchange message? I have a proxy (ASSP) in front of Exchange, but the proxy does not report any errors/issues. I can't find this exact message in ASSP so I'm pretty sure its coming from exchange 2019

[PS] C:\Users\administrator.MYDOMAIN\Desktop>Get-TransportConfig | Format-List MaxReceiveSize,MaxSendSize,MaxRecipientEnvelopeLimit


MaxReceiveSize            : 25 MB (26,214,400 bytes)
MaxSendSize               : 25 MB (26,214,400 bytes)
MaxRecipientEnvelopeLimit : 500



[PS] C:\Users\administrator.MYDOMAIN\Desktop>Get-TransportRule | where {($_.MessageSizeOver -ne $null) -or ($_.AttachmentSizeOver -ne $null)} | Format-Table Name,MessageSizeOver,AttachmentSizeOver
[PS] C:\Users\administrator.MYDOMAIN\Desktop>Get-ReceiveConnector | Format-Table Name,Max*Size,MaxRecipientsPerMessage; Get-SendConnector | Format-Table Name,MaxMessageSize; Get-AdSiteLink | Format-Table Name,MaxMessageSize; Get-DeliveryAgentConnector | Format-Table Name,MaxMessageSize; Get-ForeignConnector | Format-Table Name,MaxMessageSize

Name                             MaxHeaderSize          MaxMessageSize           MaxRecipientsPerMessage
----                             -------------          --------------           -----------------------
Default EXCHANGE                 256 KB (262,144 bytes) 36 MB (37,748,736 bytes)                    5000
Client Proxy EXCHANGE            256 KB (262,144 bytes) 36 MB (37,748,736 bytes)                     200
Default Frontend EXCHANGE        256 KB (262,144 bytes) 36 MB (37,748,736 bytes)                     200
Outbound Proxy Frontend EXCHANGE 256 KB (262,144 bytes) 36 MB (37,748,736 bytes)                     200
Client Frontend EXCHANGE         256 KB (262,144 bytes) 36 MB (37,748,736 bytes)                     200



Name           MaxMessageSize
----           --------------
ASSP Smarthost 35 MB (36,700,160 bytes)



Name              MaxMessageSize
----              --------------
DEFAULTIPSITELINK Unlimited



Name                                    MaxMessageSize
----                                    --------------
Text Messaging Delivery Agent Connector Unlimited


[PS] C:\Users\administrator.MYDOMAIN\Desktop>
[PS] C:\Users\administrator.MYDOMAIN\Desktop>$mb= Get-Mailbox -ResultSize unlimited; $mb | where {$_.RecipientTypeDetails -eq 'UserMailbox'} | Format-Table Name,MaxReceiveSize,MaxSendSize,RecipientLimits

Name                MaxReceiveSize MaxSendSize RecipientLimits
----                -------------- ----------- ---------------
U1      Unlimited      Unlimited   Unlimited
U2      Unlimited      Unlimited   Unlimited
U3           Unlimited      Unlimited   Unlimited


[PS] C:\Users\administrator.MYDOMAIN\Desktop>

According to your description above, the attcachment size is 7-10MB, and we will also need to note the size of the whole message.

As is mentioned in the official document, the Base64 encoding increases the size of the message by approximately 33%, so the value you specify should be approximately 33% larger than the actual message size you want enforced. For example, if you specify a maximum message size value of 64 MB, you can expect a realistic maximum message size of approximately 48 MB.

So here I would suggest you increase the Organizational limit,

Set-TransportConfig -MaxReceiveSize 35MB

Just like the discussion in this thread: 552 5.3.4 Message size exceeds fixed maximum message size (Config looks OK)