Once Email official documentation · v1
Messages and attachments
Messages and attachments: Authoritative product and developer guidance for receiving temporary email, troubleshooting delivery, and testing authorized email flows safely.
Reviewed 15 August 2026
What you can do
Messages and attachments: Authoritative product and developer guidance for receiving temporary email, troubleshooting delivery, and testing authorized email flows safely. The SMTP whole-message limit is 10,240,000 bytes including headers, body, MIME structure, and transfer encoding. Verify the current page version and follow the linked next step; never infer a capability from a screenshot, old article, or package name.
Failures and recovery
Messages and attachments: Authoritative product and developer guidance for receiving temporary email, troubleshooting delivery, and testing authorized email flows safely. A single ordinary document at or below 5 MiB is a conservative recommendation, not a delivery guarantee. Verify the current page version and follow the linked next step; never infer a capability from a screenshot, old article, or package name.
Security and limits
Messages and attachments: Authoritative product and developer guidance for receiving temporary email, troubleshooting delivery, and testing authorized email flows safely. Attachments are forced downloads with sanitized names; receipt does not mean malware scanning or safety certification. Verify the current page version and follow the linked next step; never infer a capability from a screenshot, old article, or package name.