You seem to not understand the sendgrid service at all. They aren't selling SMTP service, they are selling deliverability (along with tracking and metrics)
You seem to not understand the actual problem at all if you think you will be able to successfully deliver email sent directly from a local sendmail client on any random IP that lets you connect over port 25. No major email provider will accept your email coming from a residential IP.
If that's what they're selling, they're not very good at it. Sendgrid doesn't support all applicable RFCs, and they don't support spool, so, they have exactly 0% deliverability if the recipient host is down for any reason, or uses greylisting.
I've never had any issues with mail from my WiFi IP accepted. You'd normally send it just to your own server from your local laptop w/o any extra configuration, so, it's up to you how you configure your own server. I just tried sending to Gmail directly from Postfix on my MacBook, without any extra configuration, and it's been accepted by Gmail as well. Zero configuration on the laptop. Postfix and the mail CLI clients come preinstalled, and the mail just works.
You seem to not understand the actual problem at all if you think you will be able to successfully deliver email sent directly from a local sendmail client on any random IP that lets you connect over port 25. No major email provider will accept your email coming from a residential IP.