SMS carries text. MMS carries images, PDFs, audio, short video and contact cards. Attach a file in the SMS composer and K3X sends the message as MMS automatically. Your 10DLC or toll-free registration covers MMS too, so there is nothing extra to register.
Images: JPG, PNG, GIF. A photo of a quote, a screenshot, a product shot. Many carriers flatten animated GIFs to one frame.
PDFs: one-page quotes, signed agreements.
Audio: MP3, WAV, mostly for voice notes.
Short video: MP4. Keep it under roughly 30 seconds at standard quality or carriers reject it.
vCards: contact files. The recipient gets a one-tap "Add to Contacts" prompt.
Carrier limits vary, and K3X enforces conservative ones to protect delivery:
Total payload: 5 MB. K3X compresses images that are too large. Oversized PDFs are rejected with an error.
Per file: 1 MB recommended. Larger files are allowed but some carriers filter them.
Message body: up to 1,600 characters, longer than the 160 of SMS.
For anything over 1 MB, or when image quality matters, upload the file to the record's Files panel or a shared drive and text a link instead.
Open a person or company and click the SMS chip. The composer docks in place.
Click the attachment icon, pick a file, or drag it into the composer. You can also paste a screenshot from your clipboard.
Check the preview. K3X shows the file size and notes if it compressed an image.
Type a message body. A bare attachment with no text looks like spam to carriers and to people.
Click Send. K3X routes it as MMS.
When someone texts you a photo or file, K3X stores the attachment on the message in the record timeline under the SMS filter. Download it from there.
MMS spends credits from your single pool in the Texting category, the same pool as SMS, calls, AI actions and data reveals. MMS costs more than SMS per message. There is no separate SMS balance. See Settings → Billing for current usage.
US and Canada: full MMS support on the major carriers, inbound and outbound.
UK, Australia, most of the EU: technically supported but rarely used. Most people there expect WhatsApp or email for media. Send a link instead.
Elsewhere: support is spotty. Use a link.
MMS follows the same A2P 10DLC and toll-free rules as SMS. One registration covers both.
Carriers filter MMS more aggressively than SMS, especially an empty body with a stock marketing image. Always write a personal line.
Some recipients on basic plans cannot receive MMS or pay per message. For unknown recipients, a link is the safer default.
Sent but not received. The recipient's carrier or plan dropped it. Resend as a link.
Image looks blurry. Compression. Send a link for full quality.
PDF shows as "1 attachment" on iPhone. Normal iOS behavior. The recipient taps to open.
GIF stopped animating. The carrier flattened it. Use a short MP4 or a link.
Send your first SMS
International SMS: what works and what does not
A2P 10DLC registration guide
