The reason why your Email to SMS has been sent as a multi-part SMS is because your SMS content has exceeded the standard SMS length of 160 characters (70 for Unicode SMS).
Further details on standard SMS lengths and multi-part SMS available here.
If your SMS content was expected to be within these standard SMS lengths, there may be some other causes for this:
Your Email signature is being sent by default in the message,
You do not have the correct maximum message split set up in MXT, or
You have copied and pasted text from a word document where a Unicode is present.
To troubleshoot this, please see articles on: