MailKit VS PreMailer.Net

Third, Send-MailMessage has been deprecated. Try MailKit instead. To be honest, though, I'm yet to find time and test it myself.

Microsoft doesn't recommend SmtpClient for new development. We've been using MailKit for years.

Looks like Mailkit is the way to go for open source solutions.

Microsoft recommends MailKit in the SmtpClient docs

Jeffrey Stedfast is on reddit and is an ancap and works for Microsoft as a Senior Engineer. His Mimekit/Mailkit library for C# is recommended in the official Microsoft documentation for sending email instead of the one included by default.

TIL. Looks like the deprecation note recommends MailKit.

You could even do this with your own GMail Client. Design yourself some sweet UI, and use a simple library like: MailKit and wrap your own service around that, to get your emails. And maybe implement some features like writing yourself reminders for important mails, a few dashboards where you can drag around mails Kanban style.

recommends you use MailKit as alternate

If you don't already know about it, go check out PreMailer.net. It's a handy tool you can use to adjust styles to be more likely to work in email clients.

Up until now I've just been working with CSS which I was able to use in email by finding a tool that converts it to in-line style attributes (https://github.com/milkshakesoftware/PreMailer.Net)

CSS classes can be used in the generated code but CSS support in rendered HTML emails is very patchy and inconsistent. You should use a tool like PreMailer.NET to inline the CSS.