Message status icons tell you whether or not your message made it to the contact.
Messages go from Text Request through a receiving carrier to your contact. Unfortunately, we can't track what happens between the receiving carrier and the contact, but the carrier almost always gives us a status update. Our message status icons tell you what the carrier told us.
Here are all the message status icons you might encounter, and what they mean:
1. Sending — The message was sent, but the mobile carrier hasn't given us a status update. It's possible "sent" messages have been delivered and the carrier just hasn't updated us.
2. Delivered — The carrier has delivered your message to your contact. It does not mean the contact has seen or read your message. Read receipts can only be tracked with app-to-app messaging, such as WhatsApp or iMessage, not with SMS.
3. Undelivered — The carrier could not deliver your message to the contact. You can hover over a message’s Undelivered icon to see the reason, which could be:
Unreachable: The carrier could not reach the number you tried to text, most likely because that number is inactive. Basically, the carrier took your message to the front door, but no one was home to receive it. Confirm that you sent your message to the right number, and if you get the same error, try calling the number to see if it is indeed inactive.
Blocked: Your text number has either been blocked by the contact or “blacklisted” by the carrier. Blacklisted means your number raised too many red flags (e.g. by sending spam messages) and has been blocked. If your number is blacklisted by a mobile carrier, (e.g. Verizon, AT&T, etc.) you will not be able to send messages to any number in their network. At this point, it’s time to get a new number for texting.
404: This means either we could not find the right carrier to send your message to, or the carrier on record could not find the recipient. Sometimes old numbers get trashed, so it’s best to confirm you’re texting an up-to-date number. Other times, a number will fall into limbo for a few days when switching between carriers (e.g. if your recipient is moving their number from Verizon to AT&T). In this case, you might need to wait a day or two before texting them again.
Landline: You texted a landline number that cannot receive texts.
Filtered: Your message was filtered as spam. (Read more about avoiding spam filters here.)
Opt-Out: Your message could not be delivered to this contact because they have previously opted out of receiving messages from you (e.g. they've replied STOP to one of your messages). Contacts can opt back into receiving messages from you by texting in START.
Unknown: The carrier could not deliver your message, but won’t tell us why. Usually, this means your message was filtered, but sometimes even the carrier isn’t sure what happened.