Would Apple’s free iCloud SMTP service be sufficient for a newly launched website?

I wouldn't suggest you do this with a service that is not designed for automated emails. It's not only unprofessional, but can get shut down at any moment without notice.

Would better use a proper SMTP relay service like MailChimp.

@Geek

Would better use a proper SMTP relay service like MailChimp.

In many cases, registering for services isn't really an option for me. For instance, while the forum is mine, the domain name actually belongs to a friend, and the DNS resolution has to go through his Cloudflare account. Another complication is that the forum doesn't generate any revenue; using paid services would only increase the financial loss. Furthermore, relying on free SMTP services via email providers could expose the operator's personal information. I’m currently trying to figure out how to handle this—getting my server provider to open port 25 would likely lead to a long, drawn-out, and troublesome dispute. I’m at a loss as to how to resolve this email issue (and I’m concerned that using iCloud might end up compromising my Apple ID).

@fantuan Understood. I definitely wouldn't use your personal Apple ID. That's a recepie for disaster.

You've got to understand that for what you're trying to do -- send mass SMTP for free, there's a ton of spammers who are trying to do the same. Unfortunately, you get caught in that net.

You could always just not do email registration. I could imagine that your use case there's a lot of folks who want to remain anonymous and not out themselves if they're not ready. You'd need other anti-spam measures, but it's an idea.