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MailChannels Inbound Filtering enforces two types of attachment restrictions: a maximum message size and a list of blocked file types.

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Maximum attachment size

Inbound messages are accepted up to a maximum size of 40 MB. Messages that exceed this limit are rejected at the connection level and not delivered.

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Blocked file types

MailChannels blocks certain high-risk file attachment types to protect recipients from malware, phishing, and other security threats. The blocked file type policy applies to both inbound and outbound filtering and is consistent with the policies of major providers such as Gmail, Microsoft Outlook, and Yahoo. For the full list of blocked file extensions, the reasons they are blocked, and guidance on safe alternatives, see File attachments blocked by MailChannels.