Since the webserver will be the one sending the mails, its reputation is what determines how the mail is handled by the recipient. Things like blacklists, correct reverse DNS, SPF, Domainkeys, etc. all play a part in this decision. I don't think MX records are taken into account. It depends entirely on the recipient's mailserver, and you can only follow the best practices for mailservers.
Instead I would suggest relaying all these outgoing mails via a dedicated mailserver. It's much easier to ensure a good reputation for a single mailserver, especially once you have multiple webservers that you don't fully control.
If you have control over the mailsoftware on the webserver you should set it to relay via your own mailserver. Don't forget to configure your own mailserver to allow relaying from those webserver IP's. If you don't control the mailsoftware you can configure PHP's mail() command to submit mail directly to your mailserver.
You could also use a service that is dedicated to these kinds of transactional mails. They will take care of reputation, bounces, blacklists, etc. for you. Email delivery is hard work, and some companies specialize in it. They provide you with several options, such as an SMTP relay or an API to send directly from PHP with advanced delivery tracking, unsubscribe support, etc.
sendgrid.com is the one that we use, but other popular ones are postmarkapp.com or authsmtp.com. Rackspace provides a free SendGrid account up to 40k emails per month. Amazon has their Simple Email Service with a free tier for their hosting customers. Ask your hosting-provider if they offer smtp relay, or if they have a special offer with one of these companies.