Postmark 10 MB email limit: shrink a video clip before you attach it

Postmark accepts a message of up to 10 MB in total, attachments included, and it measures that size after the message has been base64 encoded. A video file that looks like 8 MB on disk can therefore be refused, so plan for a clip well under the limit and check the final size before you send.

Both facts come from Postmark's own page, attachment and email size limits, read 2026-10-03. Sume's side comes from Video trim.

What does Postmark actually limit?

The page gives two numbers, one for each direction of mail. The 10 MB limit is for what you send. The inbound figure is separate and only matters if you receive mail through Postmark.

Why a clip that fits can still fail

Base64 encoding turns binary data into text, so the encoded message is bigger than the file you started with. Postmark states that it calculates size after encoding, and it does not give a conversion table. The practical rule is to leave headroom and to test with the real message rather than with the file size shown in your editor.

The message body and any inline images also count toward the same total, because the limit is for the whole message.

Shrinking the clip with Sume video trim

Sume's video trim cuts one range out of a clip that is already on media.sume.com and returns a new MP4. A job costs $0.02. Import the source first with POST /v1/media-imports, because trim does not fetch from the open internet.

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 product loop. Sume's docs do not offer a file-size or bitrate target, so size is something you steer and then measure.

• Shorter is the strongest lever: duration can be as small as 0.2 s and as large as 900 s.
• A smaller frame is the second lever: output.width and output.height accept 256 to 2160.
• Dropping audio removes a whole track.

Check before you send

After the job finishes, download the result and read its byte size, then multiply by roughly 4/3 as a rough guide for base64 and compare with 10 MB. That multiplier is general base64 arithmetic, not a Postmark figure, so confirm with a real test send. If the clip is still too big, trim again with a smaller output rather than hoping.

For longer videos, skip the attachment altogether and link to a hosted page, as covered in embed video in email.

What to do when it still does not fit

Postmark's page does not publish a conversion factor, so the honest approach is to test. Send the real message to a test address through your Postmark server and watch for a size error before the campaign goes out. If it fails, trim again: a shorter duration or a smaller output frame changes the size more than any other setting Sume exposes.

Keep the original source in your workspace. Because trim never touches it, you can cut a second, smaller variant from the same import without paying for the import again.