Looking for hosted SMTP service for small ISP

[chiel <chiel () gmx net>]

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