What is the message size limit (including attachments) in Exchange Online Protection (EOP) standalone, and can it be extended?

Question

Hi,

I have an on-premises Exchange Server environment with Exchange Online Protection (EOP) standalone licenses as my email gateway.

My mail flow is configured through EOP connectors for both inbound and outbound emails.

I need to increase the message size limit from 35MB to 50MB (including attachments).

I have already configured the message size limit to 50MB in my Exchange Server, and it works fine for internal emails. However, when sending emails externally, I receive an error from Exchange Online (Microsoft 365):

Error Message: "AMS0EPF00000199.mail.protection.outlook.com gave this error: Message size exceeds fixed maximum message size [AMS0EPF00000199.eurprd05.prod.outlook.com 2025-03-19T16:19:57.214Z 08DD64C4E8CFEDBF]"

I checked the EOP portal, but I couldn’t find any settings to extend the message size limit. Based on my understanding, only Exchange Online (with mailbox users) allows such modifications, whereas EOP standalone does not provide this option.

I am still searching for a solution and would like to know if the message size limit in EOP standalone can be extended using PowerShell, similar to how it is done in on-premises Exchange.

Could you please confirm if this is possible or suggest any alternative solutions?

Reference link: Microsoft Documentation Exchange Online Protection limits - Service Descriptions | Microsoft Learn

Thanks

Binod Maharjan

Answer 1

Hello, @Binod Maharjan

Welcome to the Microsoft Q&A platform!

According to Microsoft's official documentation, it does mention that under an EOP standalone license, the email size limit (including attachments) is 150MB. this means that it is theoretically possible to receive and send emails that are larger than 35MB.

However, the message size limit for EOP standalone licenses is fixed and cannot be modified through configuration. If you have configured the local Exchange Server's message size limit to 50MB and still experience problems receiving messages from external sources that exceed 35MB, it may be due to other configurations or limitations. We recommend that you check the following points:

1.Use the following command in EMS to query the current mail size limit under the Exchange Online Protection (EOP) standalone license. After this, please adjust the email size limit to a value below 150MB according to your needs. It is worth noting that emails arriving at the receiver may be a bit larger than when they are at the sender, so please try to make this value as large as possible.

Get-TransportConfig | Select-Object MaxSendSize, MaxReceiveSize

Set-TransportConfig -MaxSendSize " "MB -MaxReceiveSize " "MB

2.Check your EOP connector configurations to make sure they are set up correctly to handle large messages.

3.Verify that there are no mail flow rules that limit the size of messages.

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Thank you for your support and understanding.

Best Wishes,

Alex Zhang