After some great support from wholly & rlparker, I got Thunderbird to access my DreamHosted webmail. However, I found that I was only able to receive mail. Any mail I sent was promptly bounced back as not being accepted - though I have a firstname.lastname@example.org.
And, after trying to resolve the problem through Thunderbird’s settings, I can’t even access my mail again.
I feel quite dumb. Can anyone offer some advice (besides therapy) and/or recommend a posting that walks one through the email client set-up process?