Cause Code Not Implemented for Heirloom-Mailx

I am attempting to send an message with an attachment from my RP3 to my phone, which uses Sprint. I have Exim4 setup as a MTA and the Heirloom-MailX configured so I can send an attachment. I am using gmail to send the text messages. I am able to send text messages from my RP3 to my phone without any issues. However, when I try to send an attachment (for example, a .png), my phone simply says "cause code not implemented". It does not give any other information or error code with this message. I am using the gateway for media/pictures (XXXXXXXXXXX@pm.sprint.com rather than XXXXXXXXXX@messaging.sprintpcs.com) when I send the attachment. The attachment itself is fine, as I can log directly into gmail and send an e-mail to my phone with the same attachment. This leads me to believe that the issue lies with Heirloom itself. Does anyone know how I can resolve this issue? I'd appreciate any help. Thanks!