With 3.12.4, I observe this behavior more often (but not always):

I respond to some mail. I send it, and it gets sent (according to my MTA). But evolution does not close the composer window, does not update the status of the mail that I replied to.

The main window still reacts, but it looks as if the IMAP component has disappeared: Whatever I do, I just accumulate “Actions” in the status bar, and nothing happens (no folder is refreshed, now mail is received).

Trying to quit evolution also has no noticable effect.

The logfile shows

Jul 19 19:44:11 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:44:12 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-CRITICAL **: Source ID 42207 was not found when attempting to remove it
Jul 19 19:44:13 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:44:15 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:17 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:18 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:21 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:22 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:22 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:24 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:27 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:28 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:34 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:35 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:39 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:40 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:40 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:40 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:41 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:52:48 kirk /etc/gdm3/Xsession[1308]: (process:15743): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Jul 19 19:52:51 kirk dbus[880]: [system] Activating via systemd: service name='org.freedesktop.hostname1' unit='dbus-org.freedesktop.hostname1.service'
Jul 19 19:52:51 kirk dbus[880]: [system] Successfully activated service 'org.freedesktop.hostname1'
Jul 19 19:52:51 kirk systemd[1]: Cannot add dependency job for unit systemd-vconsole-setup.service, ignoring: Unit systemd-vconsole-setup.service failed to load: No such file or directory.
Jul 19 19:52:51 kirk systemd[1]: Starting Hostname Service...
Jul 19 19:52:51 kirk systemd[1]: Started Hostname Service.
Jul 19 19:55:18 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:55:21 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed
Jul 19 19:55:23 kirk /etc/gdm3/Xsession[1308]: (evolution:13763): GLib-GObject-CRITICAL **: g_closure_unref: assertion 'closure->ref_count > 0' failed

although I’m not sure what parts correlate with the sending of the mail.

So eventually I have to use killev (which I have to use a lot lately). The mail is usually lost.

Unfortunately, this is not completely reproducible, and it might be that there is something else tripping evolution over earlier (probably in the IMAP support).

Again sorry for the not completely helpful bug report, but given the severity of the issue (sent mails get lost to me) I have to report it.