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Are they replying to a phone number? If they try to text to an email address or a non-mobile phone number then they could get this message.

What send-from address are you using that they are replying too?

How are you "texting" from your computer, what software are you using and what address are you sending the "text" to; a phone number or an e-mail address?

Mine works in both directions using the format xxxxxxxxxx@txt.att.net on the ATT network.

Strange; but if your sitting at a computer, just send it to their e-mail.

Do you have a signature in your email? Which mail format are you using, text, rich text or html.

Since you were doing it before you know it works! Something in the message is being picked up.

Try changing mail format if possible.

SG

Plain text should be fine. Are you using a signature in your email? Are you using a tag with your name or a quote in your email?

Obviously, something has changed in the way your email is being translated and that is usually the mail format.

Is there anything in the subject line of the email?

Looking on the web for your error message most say that it means there is too much information in the message. That seems to me that your email is being read as having more than you have or you have a signature or something else in the email that is being picked up and sent with your email/text that is being recognized as much larger than it really is.

Start with checking the mail format. Have your kids send you a text and see if you can reply to it. Double check your mail format and the content of your email for a signature or image file.

Good luck

Also, what email are your kids using? What OS is you computer? Have you tried from a web client instead of outlook express? If you have a gmail account, did you try sending directly from gmail as a test?