Litmus.com - Great. But has a fatal flaw in editor: easy to run out of previews very quickly.
EmailOnAcid.com - Great. Unlimited previews!
EmailPreviewServices.com - Cheaper $25/mo, but does not test Gmail on iPhone, which is a common one! So disqualified!
There’s also the powerful validity.com/everest/design-content, but it’s too expensive and complicated unless you actually need the other features they offer, besides just epreviews.
Hubspot - best email builder and CRM
LemList - nice easy to use CRM platform
Mautic - open source self-hosted email marketing platform
designmodo.com/postcards - build beautiful emails, easily
SalesHandy for email OMG, this is a must have! Or something similar. Get something similar. Know if/when the email you sent is opened and read! Know if/when a link in the email was clicked!