I tried to send a message over AIM, but the recipient never got it. Why?

Perhaps it contained one of these strings:

• <img
• <script

Both of these are tags in the Hypertext Markup Language (HTML), which AIM uses to encode messages (so as to support fonts, colors, etc.).

HTML tags that you put into the inputline are “escaped” by Adium, so that the recipient's client will not attempt to use the HTML tag; it will simply display the text of the tag. However, it appears that the AIM server drops messages containing even escaped HTML (but only those two tags—other tags work fine).

Another possibility is that contained a link to a file://… address. These addresses are intended to refer to files on the sender's or your computer. (The file need not actually exist on one end or the other. A perfectly valid file: link can refer to a path that does not exist on anyone's machine, such as  this one.)

AIM will drop any message containing a link to a file: address. On the other hand, it will allow the message if the address is plain text instead of a link. So, for example, “Check out what I just wrote:  file:///Users/me/Documents/Why-Adium-is-awesome.txt” will go through, but “Check out  what I just wrote” will not.

Can you fix it?

This censorship is server-side. There's nothing we can do about it.

What can I do about it?

Switch to an open protocol like Jabber. At present, we don't know of any Jabber servers that filter messages.

Do any other services filter messages?

Yes. AIM's sibling, ICQ, filters messages, and MSN also filters messages.