Not sure this information is exposed in SNMP but you could try something like:
https://serverurl.com/xml/status
and parse the resulting XML: for what you are looking for.
<gauge name="ram_utilization" current="8"/>
<gauge name="total_utilization" current="11"/>
<gauge name="cpu_utilization" current="15"/>
<gauge name="av_utilization" current="0"/>
<gauge name="case_utilization" current="0"/>
<gauge name="bm_utilization" current="0"/>
<gauge name="disk_utilization" current="0"/>
<gauge name="resource_conservation" current="0"/>
<gauge name="log_used" current="16"/>
<gauge name="log_available" current="312G"/>
<gauge name="conn_in" current="0"/>
<gauge name="conn_out" current="0"/>
<gauge name="active_recips" current="0"/>
<gauge name="unattempted_recips" current="0"/>
<gauge name="attempted_recips" current="0"/>
<gauge name="msgs_in_work_queue" current="0"/>
<gauge name="dests_in_memory" current="45"/>
<gauge name="kbytes_used" current="0"/>
<gauge name="kbytes_free" current="71303168"/>
<gauge name="msgs_in_policy_virus_outbreak_quarantine" current="0"/>
<gauge name="kbytes_in_policy_virus_outbreak_quarantine" current="0"/>
<gauge name="reporting_utilization" current="0"/>
<gauge name="quarantine_utilization" current="7"/>