Cannot send using Horde with Apple Mail

I's say you need to talk to work, in particular your system administrator. Find out what they can tell you.

You might want to check through the Google results (using horde servers):

http://www.google.com/search?q=horde&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-U S:official&client=firefox-a#sclient=psy&hl=en&client=firefox-a&hs=lOK&rls=org.mo zilla:en-US%3Aofficial&q=hordeserversettings&aq=2&aqi=g4g-o1&aql=&oq=&pbx=1&fp=d9008d84f286047

This is a guess, but since it appears that it is AT&T and/or Yahoo rejecting the server, they might have it on their "black list" possibly because of previous problems. If that is the case, there is nothing in Mail you can change - it is the server (horde) that is being rejected.

Are you sure about that?

I'm guessing ATT only allow emails to be sent with the correct (as in their) address so it's a simple rejection based on the hostname in the address. From what I understand it's an often used technique to assist slowing down spam.

To the best of my knowledge Horde is just a webmail interface and so is only relevant in the sense that the original poster knows they can access and send via it.

As in they refuse to support something or the advice they gave didn't help?

Email protocols are fairly tried and tested these days so if anybody at work uses a normal client outside of the work network then their settings should apply to your case. The only things I can think that might cause a typical client to slip up are if SSL is enforced and the type of authentication but these are all simple settings your IT people should know.

Where is mydomainname.com running? If you want to send mail from that domain, you must have a mail server there. Whomever hosts that domain should provide you with the connection parameters.

+I'm guessing ATT only allow emails to be sent with the correct (as in their) address so it's a simple rejection based on the hostname in the address. From what I understand it's an often used technique to assist slowing down spam.+

+To the best of my knowledge Horde is just a webmail interface and so is only relevant in the sense that the original poster knows they can access and send via it.+

Well, you're correct if it's only a webmail interface - you said your company was using it as a server, which meant the server is hosting it.

So, I think etresoft is correct - you need a valid server (and with that, you'd have a valid "from" address.)