Imports github.com/emersion/go-smtp github.com/resendlabs/resend-go/v2 github.com/sendgrid/sendgrid-go github.com/sendgrid/sendgrid-go/helpers/mail github.com/tailscale/tscert golang.org/x/net/html Standard library imports bytes context crypto/tls flag fmt io log math/rand net/mail os strings sync