For a long time, my partner could not use Mail.app full time because she has to be able to have alternate SMTP servers for work and home. She wants to be able to send an email from her "home account" by using the SMTP server at work when she is at work, and by using our ISP's SMTP server at home. The problem is that Mail.app will not let you set up two accounts with the same email address. The alternatives: to manually edit the preferences when switching from home to work, or to attempt a send to the wrong SMTP, fail and then select an alternative server. Neither of these could be described as elegant solutions.
However, with a text editor and the mail.plist file (found at ~/Library -> Preferences -> com.apple.mail.plist), you can do a little XML editing to set up an alternate account with the same email address but different SMTP servers. Here's how...
First make a duplicate of your com.apple.mail.plist file. If you don't make a duplicate and you screw up your mail prefs, forget where you read this. Use a text editor to find the account you wish to duplicate (identified after the AccountName Key), and duplicate everything between the appropriate <dict> and <dict> XML tags (inclusively).
Now locate the key AccountName in the duplicate (it should be the first line). In the following <string> line, change the name of the account to something that will reflect usage (eg myaccount - home).
Locate the key for AccountPath and change its string to a different path. For example, if it were ~/Library/Mail/POP-me@myemail.ca, append (alt) to the end. Mail will create the directory and mailboxes in the filesystem if and when it needs to.
Now locate key for SMTPIdentifier and change its string to the identifier of an existing SMTP server (should be the identifier that you have added via mail preferences window).
Next, locate the key for FullUserName and change its string value to something that will distinguish the account in the new message's account popup menu.
Finally, you will want to tell Mail to not fetch mail for this alternate account -- the original will be fetching mail; this is just so you can have use this email address with an alternate smtp server. After the Key and string pair for AccountType, add the following key and string lines:
<key>AutoFetch</key> <string>NO<string>Save your plist file and start Mail. Now when you create an email, you should see your alternate account in your Account: popup menu. This will send your email via your alternate SMTP server, but using your regular email return address.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030515232638824