Mailgun's 25 MB message cap: fit a video clip or send a link

Mailgun states that the maximum message size it supports is 25 MB, and that cap covers the whole message, attachments included. A product video of a few seconds can fit, while a long one cannot, so the choice is between cutting the clip and linking to it.

The limit is from Mailgun's sending messages documentation, read 2026-10-03. That page shows how to attach files but gives no separate per-attachment limit, so this post treats 25 MB as the single number to design against.

What does Mailgun document?

The page gives one limit for the message. It does not list a separate figure for each attachment, and it does not say how base64 encoding is counted, so a safety margin is sensible.

Two ways to deliver the video

Attach a short clip, or put the video on a page and link to it. Attaching keeps everything in the message and works offline, but it spends the 25 MB budget. A link costs almost nothing in message size and lets you show a longer video, at the price of one more click for the reader.

A rule of thumb that does not depend on any vendor figure: if the clip is under about ten seconds and you are willing to downscale it, attach it; otherwise link.

Cutting the clip with Sume video trim

Import the source with POST /v1/media-imports first, because Sume's video trim only reads clips on media.sume.com. A job costs $0.02 and returns a new MP4.

Trim the clip first. Send start and either end or duration, plus output with a smaller width and height (256-2160) and an fps of 24, 25, 30 or 60. precision has to stay exact, the default, for output to be accepted. Add audio: "drop" for a silent loop. Sume's docs offer no file-size or bitrate target, so you steer size with length, frame size and audio, then measure the result.

• Cut length with start plus end or duration (0.2 s to 900 s).
• Lower output.width and output.height (256 to 2160).
• Pass fps 24, 25, 30 or 60 if the source runs faster than the email needs.

Verify the size, then send

Download the finished MP4 and read its byte size. Leave margin under 25 MB for the message body and for encoding growth, because Mailgun's page does not say how it counts size. If the result is too big, trim again with a smaller frame or a shorter range.

For a poster image that links to the full video, video frames can pull a still from the clip at a time you choose.

A test that costs almost nothing

Because Mailgun's page does not say how it counts size, send a test message through your own domain's sandbox or to an address you control, with the real clip attached. A refusal tells you the clip is too big. A delivered message tells you it fits, but check the received size in the mail client as well.

If you send to many recipients, remember each message carries its own copy of the attachment, so a smaller clip also reduces the bandwidth of the whole send.