I've seen this exact pattern with a few local shops I've worked with. The December drop-off isn't a coincidence - Google and Yahoo rolled out new sender requirements in late 2023 that hit everyone who wasn't properly authenticated. If you're missing SPF, DKIM, or DMARC records, your emails get flagged or land in spam.

First thing: check your domain's DNS. Your email provider should have a guide on setting these records. You can do it through your domain registrar (like GoDaddy, Namecheap, etc.). A typical SPF record looks like:

v=spf1 include:_spf.google.com ~all

For DKIM, you'll generate a public key in your email tool and add it as a TXT record. DMARC is a policy record:

v=DMARC1, p=none, rua=mailto:you@yourdomain.com

Start with p=none to monitor without blocking anything.

Next, use Google Postmaster Tools (postmaster.google.com) to check your domain reputation. Likely you'll see it's low or failing. That's the spam doghouse you mentioned.

You also need to clean your list. Invalid addresses or stale ones that haven't engaged in years will drag down your sender score. Use a bulk email validator - I've used ValiDora because it charges a flat monthly fee, no per-email surprises. Remove hard bounces, obvious typos, and addresses that haven't opened in six months.

After that, send a small, text-only re-engagement campaign to your most active customers. Keep it simple: "Just checking in - we missed you." Monitor open rates before scaling up.

This isn't necessarily a 'switch tools' problem. If your current tool supports SPF/DKIM setup and you can add an unsubscribe link (which is now required anyway), you can fix this without moving. But if your tool is basic and doesn't let you manage authentication or view bounces properly, then yes - consider something like Mailchimp, SendGrid, or even a simple CRM integration.

The key takeaway: December wasn't random. Those new sender rules are permanent, so get your DNS sorted, clean your list, and monitor your reputation. You'll climb back out within a few weeks if you do it right.