If your contacts are trying to reply to your messages and receive a response like this: "From: 1011100001 Your MSG could not be DELIVERED because InvalidPduContent".
This typically occurs when you send your messages using Standard Delivery and the carrier delivers the message through their internal short code. If your contact would like to reply to the message they will need to reply to the email that is appended to the beginning of the message. This type of delivery is completely dependent on the carrier and handset and we have no control over it.
If you use Express Delivery you will avoid this problem all together because messages are sent from our dedicated short code 25827 - which always come back to our system.