Sending email functionality should check proper settings depending on whether smtp/sendmail is chosen.

1. We should not check smtp auth settings if sendmail is the mailing option

2. we should not check existence of smtp machine if sendmail is the option. we should check that sendmail path is set properly

i think we do these checks in 2-3 places. please make sure we centralize these checks to 1 place (in CRM/Utils/Mail). Please reassign the issue to me when done, so i can do a code review

Check: http://forum.civicrm.org/index.php/topic,4586.msg19985.html#msg19985 for more details