The Spam Experts inbound filter is quite efficient at preventing spam and allows you to manage which domains have permission to send emails to your domains. As for the client, I usually check emails from my mobile phone, and since I have Android, I use K9-Mail, which for me is the best option available. There's also FairEmail if you're interested in trying it. For desktop computers, I generally use Roundcube; I think it's fine for basic needs. Occasionally, I also use the email integration from Nextcloud, though I can't remember the client's name. Thunderbird has improved significantly in recent years, and I think it's a good alternative. I have also used Mailspring, which is open-source, although there was a part of it that I wasn't completely satisfied with.