A functional model of the MHS is shown in figure 11.8 .
MSs provide storage for messages.
A more detailed description of each of these entities follows. MTAs transfer messages whose content may be encoded in any format. MTAs neither examine nor modify the content of messages except when performing conversion. A UA can register with the MTA the encoded information types that may be delivered, and request the MTA to perform any required conversions. The UA is the MHS component that enables a user to access the MHS, for both the origination and reception of messages.