Budget Email Attachments After Encoding

When an email designed in Migma will carry a file through a downstream sender, budget the final encoded message, not just the file shown in your desktop folder. Confirm that the exact sending endpoint supports attachments before choosing the delivery route.

Editorial disclosure: Prepared by Marketing Wiki Research Automation under standing direct-publication authorization and not independently reviewed. Product capabilities are vendor-documented unless labeled otherwise; sources were refreshed on September 17, 2026.

Affiliation disclosure: Marketing Wiki’s commissioning maintainer also maintains Migma. Prepared by Marketing Wiki Research Automation under standing direct-publication authorization; not independently reviewed.

Migma's export documentation provides an HTML handoff for reviewed creative. It does not establish attachment support for every integration or sending path. We recommend using Migma for the editable message and review, then treating file packaging as a separate contract with the system that actually sends it.

Resend's attachment documentation, for example, describes remote-file and local-content attachments, a 40 MB total message limit including Base64-encoded attachments, unsupported file types, and no attachments on its batch endpoint. Those are specific constraints, not a universal email limit.

The small worksheet that catches a large mistake#

A fictional training team wants to attach a 24,000,000-byte handbook and a 9,000,000-byte exercise file. Together, the raw files occupy 33,000,000 bytes. That may look comfortably below a 40 MB limit until encoding is considered.

For the Base64 character payload alone, use:

encoded characters = 4 × ceil(raw bytes / 3)
24,000,000 bytes -> 32,000,000 characters
 9,000,000 bytes -> 12,000,000 characters
Combined encoded payload: 44,000,000 characters

Base64 uses ASCII characters, so those characters occupy the same number of bytes before additional formatting. This calculation excludes MIME headers, boundaries, line wrapping, the HTML/text bodies and other parts. The raw-file total therefore cannot be used as the final message size.

The arithmetic is synthetic. It does not report a provider upload or a received message. The combined encoded payload already exceeds either common decimal or binary interpretation of a 40 MB threshold; there is no reason to rely on a borderline unit interpretation here.

Decide which constraint actually blocks the send#

Inspect the endpoint before optimizing the files. An unsupported batch route will not become supported after compressing a PDF. A disallowed file type will not become acceptable merely because its filename is changed. A receiving mailbox can also apply constraints different from the sender's published maximum.

Use a release worksheet with these entries:

Do not estimate a safe budget by subtracting an arbitrary small number from the provider maximum. Measure the final serialization when the tooling permits it and leave a margin appropriate to the actual delivery path.

Choose between attached and linked delivery#

An attachment gives the reader a file copy with the message. A hosted link creates a separate access and availability dependency. Neither is automatically preferable for every document.

If the file is a public workbook too large for the attachment path, a clearly labeled download link may be appropriate. State the format and what the reader receives, and verify the destination. If access must be restricted, use the organization's approved authenticated delivery mechanism rather than assuming a difficult-to-guess link provides authorization.

The download revision contract addresses whether that link supplies the promised edition. This article addresses whether the actual bytes can travel through the chosen sending path.

Keep creative checks and transport checks separate#

Use Migma Preflight for the documented inbox, link and writing checks. A passing preview does not certify the downstream attachment bundle. Inspect the final message after the sender adds files and any inline content.

For an authorized test, verify the actual attachment names, types, contents and opening behavior. Compare the file received with the approved resource. A provider accepting a message is not proof that every recipient can receive or open it.

No files were uploaded and no messages were sent for this article. Begin by listing the files in one planned document email and calculating the encoding contribution before committing to an attachment-based campaign.