Another possibility, Many Antivirus programs scan out going mail and handle the SMTP authentication themselves. Some enable encryption by default. If the mail server does not require encryption sending will fail. So check the settings in your AV program if this is the case.
Bill
Main Box*i7 930*GA X58A-UD3R R2*3x4 gig Patriot DDR3 1600EL*EVGA GTX 460 1 gig*Intel X25-M G2 80 gig*WD Green WD20EARS 2 TB*ASUS DRW-24B3LT*Samsung SH-S223L*Corsair AX750 PSU*Rosewill Challenger case*Windows 7 Ultimate 64 bit SP1