Developing reasonable secure email sending functionality

We are developing a desktop app and want to add email sending functionality from within. The problem we now have is that the old smtp libraries are no longer entirely adequate since providers like gmail now call them 'applications that do not meet modern security standards'. They still allow you to enable less secure apps to access your email account, but the user is no doubt going to be put off by this and we want to avoid that. On the other hand, the OAUTH 2 protocol that seems to prevail requires consumer key and security to be provided by the email provider and we cannot surely collect these from every provider that our users may subscribe with.

My question is: What is the standard approach these days to email sending from within desktop applications? Can we just use good old TLS and live with the potential consequences? Surely there are loads of applications that do not implement oauth - every fax machine, photocopier or printer can send emails, it seems quite doubtful to me that they use OAUTH.

Many thanks in advance!