I'm currently using EIS (Eudora 2.1.1) to get my email through optonline.net
Problem is whenever I "reply" to emails I get the following error:

Error 5.7.1 relay

I have no problem getting messages. Its the replying that is the problem. It never leaves the outbox and I get the above message. Any ideas? Of course there's no support for this freeware product from Eudora.

Thanks in advance.

Many ISP's are making changes to their mail sending requirements to eliminate SPAM.

Your outbound SMTP address is the culprit. The relaying error that you are getting indicates that you are trying to send your message through a different outbound SMTP than your carrier SMTP. Try using your phone system's
SMTP address.