Strange e-mail problem

I am receiving the following error when e-mailing a couple members. I have verified the e-mail addresses, so I am unsure what could cause it.

error '80040211'

The message could not be sent to the SMTP server. The transport error code was 0x800ccc69. The server response was 550 Invalid recipient: <host>@aol.com

functions/functions_send_mail.asp, line 136

I am able to e-mail to gmail and hotmail, those are the only ones I tested outside the 2 in question.

Any advice to check please?

Where would you specify which addresses were allowed and not allowed please?

While you're at it, blocking hotmail and yahoo email in the signup is also a good idea.

aol, msn, and yahoo tend to use excessive spam blocking which makes it difficult to communicate with anyone using those addresses. whith msn and yahoo you'tr likrly to end up in their junk mail box. aol at least bounces it back to you so you know the recipient didn't see it.

All three services are also notorious spam sources themselves. People will sign up with a address from them just to spam your forum.