I have been working with outlook for quite sometime now (well over 10 years actually), I only found out this solution to fix our mobile user problem today. What they want to do is send/receive email on their laptop while they are on the road, it is now very easy with their USB wifi dongle. however they can’t send email, the outgoing server didn’t authenticate properly.
I tried with the setting “POP before SMTP” but very interesting, it still doesn’t want to work. I ended up finding a simple answer, change the SMTP port from 25 to 587. Apparently some ISP doesn’t let you route with their port 25, so we can only use port 587, which this port doesn’t have the encryption. Most importantly it fixed the problem and works for the users.