On running Evolution in valgrind:

==15562== 202,750 (96 direct, 202,654 indirect) bytes in 6 blocks are definitely lost in 
loss record 23,482 of 23,512
==15562==    at 0x4A0649D: malloc (vg_replace_malloc.c:236)
==15562==    by 0x3D33049600: g_malloc (gmem.c:164)
==15562==    by 0x3D3305E476: g_slice_alloc (gslice.c:842)
==15562==    by 0x3D3305F802: g_slist_append (gslist.c:254)
==15562==    by 0x19922118: get_items_response_cb (e-ews-connection.c:575)
==15562==    by 0x19923C4B: ews_response_cb (e-ews-connection.c:446)
==15562==    by 0x3D48043069: process_queue_item (soup-session-async.c:383)
==15562==    by 0x3D480432BF: run_queue (soup-session-async.c:418)
==15562==    by 0x3D48043912: idle_run_queue (soup-session-async.c:441)
==15562==    by 0x3D330427EC: g_main_context_dispatch (gmain.c:2441)
==15562==    by 0x3D33042FC7: g_main_context_iterate (gmain.c:3092)
==15562==    by 0x3D3304360C: g_main_loop_run (gmain.c:3300)
==15562==    by 0x19923455: e_ews_soup_thread (e-ews-connection.c:794)
==15562==    by 0x3D330683A5: g_thread_create_proxy (gthread.c:1955)
==15562==    by 0x3D31407B30: start_thread (pthread_create.c:305)
==15562==    by 0x3D310DFD2C: clone (clone.S:115)
==15562== 

This means that some user of the get_items call is not correctly calling g_slist_free() to free the *list* data structures.