Try to enable DebugOutput in site.ini.append.php file.
[DebugSettings]
DebugOutput=enabled
For your SMTP configuration add TransportUser and TransportPassword. Usually in settings/override/site.ini.append.php Sometimes SMTP server requires authorization.
Does eZ support SMTP authorization? If is, is it configurable somewhere?
Yes, See
http://ez.no/doc/ez_publish/technical_manual/4_0/reference/configuration_files/site_ini/mailsettings/transport
http://ez.no/doc/ez_publish/technical_manual/4_0/reference/configuration_files/site_ini/mailsettings/transportuser http://ez.no/doc/ez_publish/technical_manual/4_0/reference/configuration_files/site_ini/mailsettings/transportpassword