Who This Is For / When to Use
This guide is for users sending MMS messages in workflows who experience issues with images not attaching, failing to deliver, or Custom Values not resolving correctly at send time.
How do I attach a Custom Value URL to an MMS message?
Custom Value URLs must be created first and then inserted into the MMS attachment field inside the workflow action.
Steps:
Go to Settings → Custom Values.
Click + Add Custom Value.
Enter a clear name (for example:
appointment_confirmed_mms).Paste the direct image URL into the Value field.
Save the Custom Value.
Open the workflow and edit the Send SMS/MMS action.
Click Add attachment, open Custom Values, and select your Custom Value.
What file types are supported for MMS attachments?
Only standard image formats are supported for MMS delivery through carriers.
Supported formats:
JPG / JPEG
PNG
GIF
Files outside these formats will fail to deliver even if the Custom Value is configured correctly.
What is the maximum MMS size allowed by carriers?
Each carrier enforces strict MMS size limits. Exceeding these limits will cause silent delivery failures.
Carrier size limits:
AT&T: 0.6 MB
Sprint: 1.4 MB
T-Mobile: 1.5 MB
Verizon: 0.675 MB
For best reliability across all carriers, keep MMS files under 0.6 MB.
My Custom Value is not working when the MMS sends. What should I check?
Custom Values usually fail due to formatting issues, file size limits, or inaccessible URLs.
Checklist:
Ensure there are no spaces before or after the URL.
Confirm the file size is within the carrier limits.
Test the Custom Value by pasting it into a test funnel or preview.
Send a test SMS to confirm the image resolves correctly.
Why does my MMS fail even though the size and format are correct?
If size and format are valid, the most common cause is the hosting URL.
The URL must meet all of the following:
Publicly accessible (no login required)
Direct image link (ends in .jpg, .png, or .gif)
HTTPS enabled
Links to Google Drive previews, Dropbox share pages, or private servers will not work because carriers cannot fetch the image.
Can I use multiple Custom Value URLs in one workflow?
Yes. Multiple Custom Values can be used across different workflow steps, but each MMS attachment must independently meet file size, format, and URL requirements.
Using multiple attachments in a single message is not recommended due to carrier limitations.
How can I confirm my Custom Value is set up correctly before sending?
Testing prevents failed campaigns and undelivered messages.
Best practice testing steps:
Send a test SMS from the workflow editor.
Verify the image loads instantly on the test device.
Test delivery to at least one carrier if possible.
Confirm the image URL opens directly in a browser.
Common Issues and Fixes
MMS not delivered: Reduce image size below 0.6 MB.
Broken image icon: Replace URL with a direct image file link.
Custom Value not resolving: Remove spaces and re-save the Custom Value.
Works in preview but not live: Ensure the hosting URL is public and HTTPS.