We tried to send a client's campaigns (mostly announcements to their own customer base) to around 300 adresses, and on every single try the emails didn't go through Gmail. Not even on the spam folder. Tried contacting support, who asked to retry a few times, check the list, etc. DKIM and DMARC were all setup, yet the emails didn't arrive on Gmail inboxes. So far, no solution from support.
Sending the campaign to gmail test addresses worked, and sending it to a list of 5 gmail adresses worked too. But trying to send it to around 100 adresses, they all bounce, but you wouldn't know from their reports which almost always claimed 100% delivered and almost no opens.
So we had to send the emails manually straight from Gmail. Our usual transactional email provider works like a charm sending from the same domain, so we know it's an issue related to Acumbamails servers (possibly blacklisted as others pointed out).

We had high hopes for the agency features and unlimited subaccounts, but it is useless to us and our clients if the largest email provider isn't reachable through this platform.