Realying Problems

[Greg]

Ages ago we were added to a list because our server was open to relays. Fine, we changed this and were duly taken off it. Recently, however, another site of ours (who only use dialup) started checking their mail via pop, which worked fine. The only problem was they couldn't send mail through the server, so just temporarily, we unchecked the relaying. Few months later, we suddenly realised we were back on the relaying list! However, we still can't find a way for our other site to send mail via us. I set their mail up so the Outgoing mail server requires authentication (set to use the same incoming mail settings), which I feel should let them send mail through us! It just gives the same message about realying. Is there something really stupid I am missing? Thanks for any help!

Cheers,
Greg