If your email provider doesn't ban you for sending spam, phishing emails, etc, then you better switch providers. Otherwise the service will attract spammers and criminals, and your outgoing emails will end up in spam folders.
I agree that many of those items (harassment, copyright-infringing material, self-promotion, offensive language) are vague and could be abused by Vivaldi if they were out to get you for whatever reason. So it comes down to whether or not you trust Vivaldi to enforce their policy reasonably and fairly. If you don't, then you shouldn't use them.
At the end of the day, any service provider needs to have some discretion on these things, because the line between malicious and legitimate usage can be too blurry to express as a precisely defined policy.
That's crazily vague
Edit: to me (and I suspect most of us) pictures of women breastfeeding infants are not "pornography", but you still hear about such images being banned all the time.
1. whether the average person, applying contemporary "community standards," would find that the work, taken as a whole, appeals to the prurient interest;
2. whether the work depicts or describes, in an offensive way, sexual conduct or excretory functions, as specifically defined by applicable state law; and
3. whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.
Of course, but that's not the issue under discussion.
The issue under discussion is the combination between (a) how vague these things are (b) the extremely wide range of badness (don't tell me that "self-promotion" falls into the same category as "spam/scams") and (c) the fact that your email account can be locked if you violate these rules on the forum.
It's one thing to claim that you're closing someone's forum account because they were harassing someone on it, and another to close their email account. That's not protecting your forum, that's trying to give yourself the ability to legally retaliate against someone for anything that they did that you don't like.
The issue with humans, is that whatever policy you come up with, there's going to be someone who reads the policy and skirts just around the edges. For example, they could say "no unsolicited marketing" and "no impersonation", which seems like a reasonable enough policy. But then you can imagine a user who habitually responds to support question on the forum with: "Great question, I can figure that out for you, email me at paid.consulting@vivaldi.com for a quote." It's not unsolicited per se, and not impersonation per se, but I can see why Vivaldi would want the discretion to ban this hypothetical user and disable his email address.
"Don’t use sarcasm and jokes that can be misinterpreted."
in their CoC. This makes perfect sense of course, we wouldn't want to get mixed up with the dreaded sarcasm-users.