Mailgun configuration to improve the sending of emails

In our VPS we have problems with the delivery of mail: Some arrive at the SPAM tray, others do not arrive and others simply bounce. And these bring us many complaints from our customers. Hello, In our server we have problems with the delivery of mail: Some arrive in the SPAM tray, others do not arrive and others simply bounce. I often receive calls telling me that: - The emails they send do not reach Hotmail. - They send emails and receive an error message. - The sent emails arrive in the SPAM tray in Gmail. I heard that this can be improved by hiring an email delivery service. For that reason I have decided to contract some external service for the emails that are sent from my server. The first option I found on the internet was MAILGUN, but I do not know if it will be the right one or the best one. For that reason I need: Advice on the best or alternative way to hire an external email service since I want to improve the delivery of emails. Integrate this external service in our server to improve the delivery of emails. Keep in mind that on my server there are multiple client accounts, who access their emails by [login to view URL] (see how the solution is adapted). A cordial greeting. Erick Mines L.