<domain> does not accept mail (see rfc1846)
The recipient mail server does not actually accept and deliver email itself. From this point, emails can either fail or be relayed/forwarded to another mail server. Most commonly, this is shown for relays.
If you received this error message in your inbox, it means that you tried to send someone at AOL an email message, and AOL’s mail server rejected the connection because it’s seen just a bit too much spam lately from your mail server. Ref: 1)AOL 521
521 5.2.1 : (CON:B1) http://postmaster.info.aol.com/errors/554conb1.html