HI,
As an @mac.com name and in fact an @me.com name as well are valid AIM Screen Names then you will get a messages from AIM if the password is "wrong".
The most common reason for it being "wrong" is, as I said in my first reply, that Apple encourage and allow longer passwords than will work with the AIM servers.
@mac.com and @me.com are different.
@mac.com names originally came from registering for the then .Mac service. (as an iChat name was later)
Later this was replaced by the MobileMe service that issued the first of the @me.com names.
When MobileMe replaced .Mac those people that had paid for accounts with .Mac were offered the choice about whether to keep their @mac.com name "Alive" by Linking it for email purposes to their new MobileMe name.
(This only worked in the Mail or other apps that could access mail. Effectively you had one Account but it had two emails names. For iChat and AIM logins they worked as two separate names).
At this time the @mac.com names registration point became a place for "iChat Names"
More recently Apple have essentially stopped MobileMe (The last paid for accounts will run out on the 30th June) and have replaced this with iCloud (at least as far as an Email address and registering for same with Apple). It still uses the same @me.com names. (for iChat purposes this @me.com version will only work in iChat 6 and above)
iCloud names can be created using any existing Apple ID (Older @mac.com and @me.com as well as any other email used as an Apple ID - this does in fact do a sort of Hard Linking of the two names much like the original MobileMe and @mac.com names at the change over.
Summary (so far)
@mac.com and @me.com names issued by Apple are also valid AIM names that can login to the AIM Service.
The Password has to conform to the AIM server limits for this.
For iChat purposes @mac.com and @me.com names are two different Screen Names although at the .Mac service and MobileMe change over they could be linked for email purposes.
There has been a subtle change with the start of iCloud issued @me.com names that causes them only to work in iChat 6 or higher.
(you also have to login to me.com so that the AIM Servers can "See" the password and only iChat 6 and Messages beta does this).
Next.
You will need this Users Tip which explains in the lower "Next Step" section how to check what your Computer thinks is the password to the @mac.com name.
Whilst checking this in Keychain Access also check the Access tab of the item to check that iChat is allowed to access this password. If not it will tend to send a blank password which will get rejected by the message.
It will also pay to check the AIM Settings site listed in the upper section of the Users Tip to see if the name and password you think it is works.

8:10 PM Saturday; May 19, 2012
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