We use postmarkapp.com for our transactional and marketing outbound. It works extremely well and has excellent diagnostics. We've been with them a few years.

-----Original Message-----
From: NANOG <nanog-bounces+ray=oit.co@nanog.org> On Behalf Of John Levine
Sent: Thursday, January 14, 2021 6:06 PM
To: nanog@nanog.org
Subject: Re: Looking for hosted SMTP service for small ISP

In article <670aea8b-ef34-6450-32f1-725ce6de7e96@gmx.net> you write:
>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.

Is this really outgoing only, or are you also providing mailboxes for incoming mail?

An obvious question if you're not handling incoming mail is why don't they use their own mail provider (the one that handles mail for the domain on their From line) as their outgoing host?

If you're looking for mail hosting at modest scale, I've been pretty happy with Tucows white label service.

If it's outgoing only I would not recommend sendgrid who are chronically unable to manage their compromised and spamming customers.
smtp.com might be a little better.

R's,
John