Why can I send email from my iPad, but my iMac which is connected to the same modem keeps telling me that it can't send the message using the same server?

Question

I have an iPad 2, connecting via Wifi to an Actiontec Q1000 modem.  The iMac (3.06 GHz Intel Core 2 Duo, 4 GB 1067 MHz RAM, running OS 10.8.2) is also hard wired to the same modem.  Internet access is via CenturyLink.  Main email is from Shaw cable in Calgary (Phoenix is a vacation property), not through CenturyLink.
Can receive email on the iMac, but every time I try to send, I get a pop up saying "Cannot send message using the server .......".  Further prompts me to verify the address in the email, and SMTP settings in Mail preferences, or to select a different outgoing mail server.  I've tried using a number of servers, always the same result.
Using same settings in both devices.

Answer 1

Mail is not my forte, but what I noticed was that you have Centurylink and presumably have your Mail set up with their settings? But then you mention Shaw/vacation property/etc - you can send out email only through the server that you have set up as an account using the ISP you are using. If you switch places, do you have separate accounts set up with both Centurylink and Shaw so you can switch and use whichever one you're at? A Phoenix ISP won't work with a Mail POP account unless you're in your home - Phoenix - area.
Do you have a data plan with your iPad? That would explain why that works - it'll function sort of like a cellphone - works anywhere where there are signals.