Looking for hosted SMTP service for small ISP

Hello, We are a small ISP. Most of our customers have a hosted e-mail service (gmail/office365/etc..) and have their own outgoing email sorted. But a handful of customers rely on our SMTP server for outgoing e-mail. Currently we host this our self with a physical box. But I would like to have a hosted solution so that I don't have to worry about keeping up with updates and latest spam techniques. Preferred I'm looking to make a DNS name like smtp.example.com and point this to a hosted solution. This excepts any email from our IP ranges. All controlled with a nice dashboard. I was in contact with Solarwinds (Mail Assure) but they don't seem to be able to help. Chiel