Posteo is great for business. It's a professional and competent company that respects your privacy to a good degree, and has a clue about security. In a sense it's MORE professional than trying to DIY, unless you really know what you're doing. I would feel encouraged to see a business use Posteo when I receive the email. It tells me that your business probably cares a little about security and/or privacy and possibly the environment too. Total kudos for using Posteo from me.
What I think is completely unprofessional is to use your ISP's address for business email. The number one unprofessional business move, in my opinion, is to have your business website on Facebook/Meta, Twitter or any other enshitified corporate social media site. You won't get my business. I'll go anywhere else no matter how inconvenient.