The iOS version of Sparrow has just been released, and it contains this functionality. In the settings for email accounts you can set up aliases. You can also set up a custom signature and SMTP server for each alias:
When composing a message, by default it will be sent from your default alias. If you want to change this, tap the "From" in the top center and you will be able to select which alias you want this message to come from:
Sparrow is available in the App Store.
There is no need to log in to individual accounts, at least with more modern versions of OS X. You can set access privileges for managed accounts from within the Users & Groups pane of System Preferences without leaving your own account.
By selecting the Other tab in the Parental Controls dialog you can even enable Remotely managing parental controls.
Upon further clarification by Cra, it seems that Apple's file quarantine system is causing the difficulty. Starting with Mac OS X 10.5, files downloaded from the internet are quarantined and checked for safety.
Applications that are downloaded must be approved the first time that they are run, and only the owner of the account used to initially download the software can grant that approval.
If you have multiple user accounts on your Mac, the user account that downloaded the file is the only user account that can remove the quarantine attribute to the file. All other user accounts can open the quarantine file, but they will be presented with the quarantine dialog box asking "Are you sure you want to open it?" every time they open the file.
ᔥ Apple Knowledgebase:About file quarantine in Mac OS X v10.5 and v10.6
Make sure that you download software updates using your administrator account and that you open and approve the software from your account before making any adjustments (which may not be necessary at this point) to the Parental Control settings. It may be necessary to disable automatic updates in Firefox.
The quarantine system works by attaching an extended attribute to a downloaded file. If absolutely necessary, you can remove this attribute using the terminal
Enter the following command, replacing appropriately:
xattr -d com.apple.quarantine /applicationPath/<applicationName>.app
See also: Stack Overflow: How do I remove the “extended attributes” on a file in mac
Best Answer
I had a somewhat similar question about iOS a few months ago and got an excellent recommendation for postfix -- for osx. Unfortunately that answer did not apply to iOS but it may be a useful answer for your purpose!
https://apple.stackexchange.com/a/102708/52358
You want to take a look at the answer by buscar