I've not spent much time looking at Eudora, but almost any mail client except Outlook and Outlook Express should be much safer from Viruses, simply because they don't have enough market share to justify the effort. But there's more than just viruses you need to be aware of when choosing a safe email client.
I refuse to use a mail client which automatically fetches referenced images in HTML email. Embedding image references which trigger server-side processing is a favourite spammers' tool for validating that junk email has been opened. Before you settle on an email client, make sure you can turn this insidious feature off. Then, when you have turned off automatic image fetching, look at some example HTML email and see if it still readable. There are always some misguided fools, managers and the like who will insist on sending HTML email, so you need to be able to read it without dropping your trousers.
Make sure that any email client you choose can export messages, indexes, contacts etc. in various formats as well as just import them. Your contacts and mail archive are very valuable, so it's vital to be able to take them with you, even if your email client becomes obscure and effectively unsupported.
Where's this all leading? A suggestion that you check out TheBat! from
Ritlabs. I now recommend this to all my clients who are worried about these sort of issues.
