I am a Windstream ISP user with Apple Mail on iOS 14.3 on iPhone 12. All emails I send that forward anything get blocked and placed in my Outbox. Only when I turn off SecureVPN will they send. Also lots of issues connecting to links in emails.
Email can be problematic any time you use a VPN. The email provider expects an IP address that represents your actual location. When it sees a different location, it blocks your account to protect it against possible hacking. Contact your email provider to see if they can offer any work arounds.
As for the links in emails, what issues are you seeing? What error messages do your see? Do these issues also go away if you turn off the VPN?