SMS carries text. MMS (Multimedia Messaging Service) carries images, audio, short video, vCards, and PDFs. From K3X you attach a file the same way you would in iMessage — K3X figures out whether to send it as SMS or MMS based on what's in the message.
What works.
Images — JPG, PNG, GIF (animated GIFs are flattened on most older carriers). Photo of a contract, a screenshot of pricing, a product image.
PDFs — up to a few MB. Common for one-page quotes or signed agreements.
Audio — MP3, WAV. Mostly used for sending voice notes.
Short video — MP4. Tight size limits (see below) — anything more than ~30 seconds at standard quality will get rejected.
vCards — contact files. Recipient gets a one-tap "Add to Contacts" prompt.
Size limits.
This is where MMS gets fiddly. Carrier limits vary, and K3X enforces the most conservative ones to maximize delivery:
Total payload: 5 MB max. K3X auto-compresses images above this. PDFs over 5 MB are rejected with an explanatory error.
Per-file: 1 MB recommended. Larger files are technically allowed but get filtered by some carriers.
Up to 10 attachments per message. But just because you can doesn't mean you should — multi-attachment MMS has worse deliverability.
Message body: 1600 chars (longer than SMS's 160; the extra space comes free with MMS).
If your file is too big, K3X offers to upload it to its file-share service and send a link instead. Recipients click the link, which opens a branded download page. This is the recommended path for anything over 1 MB or anything where image quality matters.
How to attach in K3X.
Open the SMS composer on a lead.
Click the paperclip icon.
Pick a file from your computer, or drag-and-drop into the composer.
K3X shows a preview with the file size and a compression notice if it auto-shrunk an image.
Type your message body (recommended — don't send a bare attachment, it looks like spam).
Hit Send. K3X automatically routes as MMS.
You can also paste images directly into the composer from your clipboard — useful for screenshots.
Pricing.
MMS costs more than SMS:
SMS: about 0.8 SMS credits per message.
MMS: currently billed like a regular message — about 0.8 SMS credits — regardless of file size or count.
Pulls from the same SMS credit bucket. The 200 SMS credits included each month cover roughly 250 messages, whether SMS or MMS.
Inbound MMS.
When someone sends you an MMS, K3X downloads the attachments and stores them on the lead's activity feed alongside the message. Attachments are scanned for malware before being made available; suspicious files are quarantined and you get a warning.
You can download inbound attachments individually or as a zip. Retention follows the same policy as the parent SMS thread.
Carrier and country support.
US/Canada: full MMS support on all major carriers, both inbound and outbound.
UK/Australia/most of EU: MMS is technically supported but rarely used — most senders rely on WhatsApp or RCS instead. K3X will fall back to sending a link if the recipient's carrier doesn't accept MMS.
Other international: MMS support is spotty. K3X auto-falls back to a hosted link in unsupported regions.
Compliance notes.
MMS is bound by the same A2P 10DLC and TFV rules as SMS. Your registration covers both — you don't register MMS separately.
MMS messages count against your throughput rate limit the same as SMS (each MMS = one outbound message regardless of how many attachments).
Carriers filter MMS more aggressively than SMS, especially if the body is empty or the attachment looks like a stock marketing image. Always include a personal message body.
Some recipients on basic plans can't receive MMS (or pay per message). For unknown recipients, a hosted link is the safer default.
Troubleshooting.
"Sent but they didn't get it." Most common: recipient on a carrier or plan that doesn't accept MMS. K3X status will say "sent" because we delivered to the carrier, but the carrier dropped it. Switch to a hosted link.
Image looks blurry. Auto-compression. To preserve quality, send via hosted link instead.
PDF didn't open on iPhone. Some iOS MMS clients show "Attachment: 1 File" without rendering inline. Recipient has to tap to open. This is normal iOS behavior.
Animation in GIF is gone. Many carriers flatten GIFs to a single frame. Use a hosted link or short MP4 video for animation.
Next: International SMS — what works, what doesn't →
