A description of the error -> "The other type of "554 Relay Access Denied" error is a problem with the sender using the wrong mail server to send their message:
Essentially, this error message is similar to a Recipient Address rejected, in that the server reporting the error does not like the domain name; however, in this case, the problem is that the outgoing email server did not like the domain of the sender's email address (as opposed to the above Recipient errors, where the incoming mail server does not like the recipients domain).
What this error generally indicates is that the outgoing server you're using (usually the one assigned to you by your ISP) will not accept email with the From: address you've used on the email. For example, domains such as BellAtlantic.com, GTE.net, and Verizon.com do not allow users to send email their outgoing mail server if the "From" email address used on the message is not the one assigned by them (e.g. you can't send out mail that uses your Pen Publishing address as the From: header via their mail servers).
Many ISPs have begun to enforce these restrictions to prevent spammers from using the ISP's access and SMTP server to send spam. To remedy this error, you will need to use the email address assigned to you by the provider as the From: address on outgoing"