How to Get Exchange Online Email Attachment Report

OneDrive and SharePoint Online serve as file-sharing hubs that enable extensive collaboration. Microsoft Outlook, on the other hand, is intended for professional communication and has a limited message size (35 MB by default) that includes both content and attachments.

However, in situations where external sharing is disabled across SharePoint Online or OneDrive, users often rely on Outlook to share files with external partners, stakeholders, or vendors. In such cases, you may need to increase the mailbox message size limit (up to 150MB) to ensure attachments can be sent successfully.

Configure message size limit for a mailbox in Microsoft 365

• In the Exchange admin center, navigate to Recipients»Mailboxes, and select the desired mailbox.
• Go to the Mailbox tab and locate Mail flow settings, then click Manage message size restriction under Message size restriction.
• Enter the value as per your requirement in the Sent messages maximum size (KB) field and the Received messages maximum size (KB) field. 
• Click Save and wait for 5 minutes to make the changes take effect.

To manage message size restrictions for a user via PowerShell, run the following cmdlet after connecting to Exchange Online.

Set-Mailbox -Identity "<MailboxUPN>" -MaxSendSize "<Size>MB" -MaxReceiveSize "<Size>MB"

Manage message size restriction for multiple Exchange Online mailboxes

• In the Exchange admin center, navigate to Recipients»Mailboxes, and choose the required mailboxes (all mailboxes if required).
• Then, click Mailflow setting, and select Message size restriction.
• Enter the value as per your requirement in the Sent messages maximum size (KB) field and the Received messages maximum size (KB) field. 
• Click Save to apply the changes.

To configure the message size limit for multiple users using PowerShell, first create a CSV file with the list of mailbox's User Principal Name as shown here.

Then, replace <InputPath> with the CSV file location and run the following cmdlet to configure message size restrictions for multiple users.

$mailboxes = Import-csv -Path "<InputPath>"
foreach ($mailbox in $mailboxes) {
    Set-Mailbox -Identity $mailbox.EmailAddress -MaxSendSize "<Size>MB" -MaxReceiveSize "<Size>MB"
}

To configure the message size limit for all existing mailboxes, run the following cmdlet with the appropriate values.

Get-Mailbox -ResultSize Unlimited | Set-Mailbox -MaxSendSize "<Size>MB" -MaxReceiveSize "<Size>MB"

When provisioning new mailboxes, Exchange Online applies a default message size limit. However, these default settings may not fit specific organizations. In such cases, admins can modify these limits using the Exchange admin center or PowerShell.

Modify default message size restrictions for new mailboxes using EAC

• Navigate to Exchange admin center»Recipients»Mailboxes.
• Then, click Mailflow setting, and select Message size restriction. 
• Enter the value as per your requirement in the Sent messages maximum size (KB) field and the Received messages maximum size (KB) field.
• Click Save and wait for 5 minutes to make the changes take effect.

Increase message size limit for new mailboxes using PowerShell

If you want to modify the default message size for new mailboxes using PowerShell, run the below cmdlet after connecting to Exchange Online.

Get-MailboxPlan -ResultSize Unlimited | Set-MailboxPlan -MaxSendSize "<Size>MB” MaxReceiveSize "<Size>MB"

Unsafe email attachments can put your organization at risk by spreading malware and other harmful content. You can make configurations to block risky file types in Exchange Online and automatically quarantine those emails before they reach users. To implement this, you can configure custom file type blocking either through the Microsoft 365 Defender portal.

Create an anti-malware policy to block specific file types in Exchange Online

While the anti-malware policy allows you to block specific file attachments, mail flow rules (transport rules) offer a broader range of actions. These rules help inspect messages to identify attachments and take appropriate actions as needed. Here are some common scenarios where transport rules can help:

Create mailflow rule to inspect email attachment in Exchange Online