How can i send mails from Dreamhost?

I can send email from webmail to a Google account of iCloud account and they will receive it just fine. I can’t send it to another Dreamhost account or back to the same account through webmail. I am getting the same results sending through iOS and sending through any desktop mail application. I would think if this was something where I screwed up settings (which I’ve double and triple-checked), then I would at least be able to send email from Dreamhost’s own webmail, but that isn’t even happening. What’s going on here?