Infusionsoft, ExactTarget, Constant Contact, Mailchimp, homegrown Postfix-based solution. Have and still occasionally use Sendgrid/Mandrill/SES as needed.
The fact that Infusionsoft uses Marketo never fails to amuse me.
so I was Infusionsoft' Infusionsoft Admin / Marketing Automation Architect at that time and then scaled the Marketo instance. This was in 2015, so things could have changed but for context- their system is for companies with: 1-50 employees, $50k - 1.5mil in revenue, db size of 1 - 100k.
Through customizing the internal Infusionsoft (and eventually connecting to Netsuite), it scaled until there were 500-600+ users, $60 million in revenue and database size of 1mil+. At that time, there were issues running it so far outside of the intended use. The company decided it didn't make sense to have the software engineers working on something that wasn't going to help the customers - and looked at mid-market / enterprise tools.
Oh, I'm totally aware of Infusionsoft's limitations and hacking around it—my company was the one that was waaaay too big for Infusionsoft's target company size that was using it for early automation and ecommerce management (but had to build a custom connector to not use Infusionsoft's in-house CRM). Not sure if they were still a customer as of 2015 but managing that thing was a beast as a result. I just find it a bit funny that there's a not-eating-its-own-dog-food component to the setup.
Which is probably for the best, as Marketo does a far better job in that space.
Began with an ESP (MailChimp) and then began marketing automation with Act-On. Transitioned from Act-On to Marketo.
As a straight-up ESP they were top notch and of late have been adding in some light-weight automations (useful for an ecommerce site perhaps).
I think transitioning from MailChimp's slick design and consumer-level ease of use to circa 2011 Marketo was especially tough, until I figured out the power under the hood.
Over the last 12 years (in chronological order):
1. Vertical Response
2. Constant Contact
3. Email Labs (dont ask)
4. Exact Target
Before Marketo starting with the very first email provider up until Marketo are:
2. Exact Target
3. Eloqua E9
4. Eloqua E10