Mmh, if you're using a custom setup that probably has to do with your Postfix/etc. configuration. They may keep and relay your original IP by default, but you can change that. Mail server configuration are complex though, and today I'd really say it's not worth the hassle of trying to run your own mail server.
I use Google Suite via SMTP to handle e-mail delivery on my servers. They have a limit of around 2,000-10,000 e-mails per day (depending on how you have it set up), which is fine for the vast majority of people, but if you send more than that you'll need something like Sendgrid.
GSuite is wonderful though and my recommendation, even with its more tricky setup. If you want something easy, Sendgrid is basically plug-and-play, but I dropped Sendgrid because they were having deliverability issues for me. Google has had absolutely zero.