Short bio: we are a CRM platform; one of our features is that customers can connect their email accounts (Office 365, GSuite, arbitrary email accounts via IMAP) and then send group messages on behalf of these accounts. That is, when a customer directs our platform to send a group message to N recipients, we would connect to their email server via API (in case of Outlook/Gmail) or SMTP (for other servers) and send a message to every recipient. We NEVER send emails from our SMTP server – everything goes through customers' servers.
We have been doing it for years, and everything was okay, but recently many outgoing messages (regardless of kind of a sender) are being marked as SPAM by Outlook Exchange, Gmail, AOL, Yahoo, etc. Not all of them but many of our customers have started to complain. We haven't made any changes on our side; we're quite confused about how to fix it. Of course, we spent time googling around, but there is so much information to get lost in easily.
My question is, are there services/platforms (something akin to https://senderscore.org/ – at least I see their advertisement) that can help us understand what happened and how to find a way out of this? Basically, a place where we can get access to a pool of experts.