We do a lot of opt-in email for the newsletters we run -- most software (such as MailChimp) that provides the services we need is very expensive, especially if

We do a lot of opt-in email for the newsletters we run -- most software (such as MailChimp) that provides the services we need is very expensive, especially if you have more than 100k subscribers. Hence, hosting the email ourselves saves a siginificant amount of money, though at the risk of not being particularly user friendly and also requiring expertise in maintaining the server and email deliverability.