Google 421-4.7.30 email response - am I for sure a "bulk sender" to them, if I get this?

I do volunteer tech work for a local foodbank and recently it seems like no one with Gmail has been able to receive our messages for the past couple weeks. I looked in /var/log/mail.log and it's filled with messages like

>status=deferred (host alt1.gmail-smtp-in.l.google.com[209.85.202.27] said: 421-4.7.30 Your email has been rate limited because DKIM authentication didn't 421-4.7.30 pass for this message. Gmail requires all bulk email senders to 421-4.7.30 authenticate with DKIM. 421-4.7.30 421-4.7.30 Authentication results: 421-4.7.30 DKIM = did not pass 421-4.7.30 To set up DKIM for your sending domains, visit 421-4.7.30 https://support.google.com/a?p=turn-on-dkim

I do know [that yes, once you hit 5,000 emails sent per day, you become a bulk sender in Gmail's eyes, permanently](https://www.valimail.com/blog/who-is-bulk-sender/). Our email list is like, 200 people so it seems bizarre to me that we'd hit that - unless we were being used as a spam relay by something bad. I don't think we were, but maybe we were. My mail logs don't go back far enough to check.

We have had SPF set up for years, but never have had DKIM.

**All I'm asking is, is that if I'm getting this return message from Gmail when we try to send a mail to Gmail, is are we for sure, irrevocably, a "bulk sender" to Gmail?**

I can't find any way on Google's own tools to pop in my website's IP or domain name to see if we're listed as one. Wish they'd just let me find out instead of letting me be unsure. I'm not going to try to argue with them if we are, as I know we'd lose, I just want to know if we for sure are, which means I need to figure out how to set up DKIM (I've done SPF many times as it's easy, but DKIM requires daemons running, etc - seemed a hassle, and non bulk-sender messages required only SPF **or** DKIM, not both.)

Last thing that may be important - I checked spamhaus.org and our webhosting IP was listed there in a PBL. We're hosting with Digital Ocean and rent an ipv4 address from them; I had that removed no problem this morning. Maybe it will just take a couple days for that removal to take effect - perhaps that was also affecting things?

edit: it looks like I'm currently back in business, I added a few test/temp gmail addresses to a second mailing list and have been trying to email them every couple of hours via the website's postfix system. Now I'm getting the happy 250 messages. I didn't add DKIM or anything ([was planning on just following these instructions for OpenDKIM](https://easydmarc.com/blog/how-to-configure-dkim-opendkim-with-postfix/) later today) so... maybe it was just waiting for Google to check with Spamhaus?