You don't need an email client to use Relenta - it has integrated webmail (which is excellent, BTW). We had no problem setting it up to read email from both Yahoo! and Gmail accounts, plus several others. At this point we're picking up email from five or six POP and IMAP accounts. All you need is three pieces of information: 1. Server/host name, 2. Username, and 3. Password - the same as setting up desktop email client - this shouldn't be difficult at all.
You can also add several SMTP accounts to send email, but our outgoing email volume isn't huge so we're OK with just one SMTP for now.
Our experience with Relenta support was superb, they're knowledgeable and responsive, so why not give it a shot?