If you plan on sending from branded company accounts for the long term, then I suggest building up your own mail servers, and setting up the whole sending infrastructure.
But, if you want to send from Google accounts, there are a few good mail mergers that will help you achieve just that.
There's a guy that created a tool called GBeast somewhere here on the forum. Haven't used it, but heard good things. Supports proxies as well.
If you need cheap sending accounts and domains, check out Mail Forge. They will set up everything for you, and the prices are astonishingly cheap. Aim for 3-5 sending accounts per domain.
PS. Dont' use transactional email providers for sending out cold marketing. You'll get banned, as cold emailing can damage their IP reputation and they won't hesitate to remove you.
Cheers