Deliverability is whether your email actually reaches the inbox, which is separate from whether anyone opens it. It depends on your sending domain setup, your authentication records, your bounce rate, and how often people mark your email as spam. When it slips, open rates fall for reasons the copy cannot fix.

Google defines a bulk sender as any domain sending close to 5,000 messages or more to personal Gmail accounts in 24 hours, and once a domain crosses that threshold the status sticks permanently. Those rules took effect in February 2024, Yahoo matched them, and Microsoft added its own in May 2025. Since November 2025, Google rejects non-compliant mail outright rather than slowing it down.

Stay under 0.1 percent, and treat 0.3 percent as a fire alarm. Hard bounces matter just as much, since a rising bounce rate is usually the first thing to pull a whole sending domain down. Both are compliance numbers now, worth checking on every send before you look at engagement.

Use a separate sending subdomain, something like email.yourcompany.com, kept apart from the subdomain hosting your website and landing pages. That protects your main domain's reputation if a campaign goes sideways. HubSpot blocks the same domain from doing both and will ask you to pick another.