SMS Spam Filters: Why Carriers are blocking your bulk messages


We get a lot of questions about the right way to use SMS marketing and send bulk messages to mobile phone numbers, and most of them have to do with the blocks or filters that carriers impose to solicited (and non solicited) messages.

In this post we try to shed some light on the subject so you can be better prepared before you decide how to use SMS messages in your company. Enjoy!

Why are my SMS messages being blocked or filtered by carriers?

People don’t like to receive unsolicited messages (aka SPAM). When they do, they will probably complain to their carriers, which in turn will act upon the request by filtering out those messages for others as well, probably fully blocking the source number too.

Carriers have a higher cost for dealing with complains and the high traffic coming in from SPAM messages, so they don’t like it as well. They invest millions of dollars per year in people and software with the sole purpose of blocking SPAM messages.

Lawyers, on the other hand, will actually be delighted if you send unsolicited messages and will surely try to put a class action for TCPA violations against you or your company, for a minimum of $500 per message sent.

Also, if you are using long codes to send bulk messages, it is quite probable that your messages will be filtered and will not be received by the targets. And in this case, it doesn’t matter if they are solicited or not. We briefly discussed this in “Short Codes or Long Codes? Benefits and Drawbacks for your SMS Campaigns

Will my SMS messages be blocked?

Carriers will always reserve the right to block specific kind of messages or numbers. And they do so based on their own and different criteria on a daily basis.

They do this as a routine process in order to protect their users and enhance their own customer experience (i.e: less spam, less complains).

How carriers filter SMS messages?

Each carrier has its own rules, and they will keep them a secret. They will also silently discard SMS messages classified as SPAM without further notice as well, returning a successful delivery status for them just to make it a bit harder to know that you are being blocked.

We can’t really tell for sure if a given message was filtered by SPAM, or why or how it was filtered. And also, the systems that classify messages as SPAM can be really smart and advanced, so each day will be a little bit harder to bypass them.

Can I know if my number is blocked or my messages being filtered?

Generally speaking, no. There is no certain way to know that your messages are being filtered.

Having said that, we do sometimes get reports from our carriers informing that a specific number is being blocked by one or more carriers. When we get these reports we will let you know with a SPAM notification.

Can I avoid being filtered or blocked by carriers?

There are some very basic guidelines that can be followed to reduce the chances of being blocked. Try that your messages:

  • Are not too long.
  • Generate MOs (replies, or inbound messages). If you can make your targets reply to the message, this will help a lot.
  • Don’t have a lot of capital letters.
  • Don’t have any suspicious links.
  • Don’t use aggressive language.
  • Include opt out instructions, so people don’t feel like the only way to opt out is to call their carrier to block the number (see our post “Automatic detection of STOP words in DIDs (aka long codes)“).
  • Use best practices for your marketing campaigns. We have explained a few of them in “Best Practices For Running Effective SMS Campaigns“).


What can I do if my number gets blocked or blacklisted?

There’s nothing you can do, except to wait. Carriers will eventually remove your number from their blacklist after a period of time. This period is not known actually, and will surely depend on how many messages you sent and when you stopped sending messages.

— The PortaText Team.