Circle uses HubSpot for marketing email and Mailgun for transactional/notifications. They rank #18,970 in the Tranco list of most-popular domains. We've also observed sends via Substack. Email goes out from 4 different subdomains. Their email authentication setup is good (score 73/100). Missing: DMARC enforcement.
Authentication signals observed in this brand's emails. SPF, DKIM and DMARC are the standard sender-authentication stack — see the DMARC policy below for whether spoofing is actually blocked or just monitored. One-click unsubscribe meets Google & Yahoo's bulk-sender requirements.
SPF / DKIM / DMARC above reflect the last 10 emails we've observed. DMARC policy is read live from DNS.
DMARC reports sent to cloudflare.net. Policy
p=none is monitor-only — receivers won't act on spoofing yet.
SPF alignment: observed emails use a third-party return-path (bf54x.hubspotemail.net) — SPF passes there but doesn't align with circle.so, so DMARC has to rely on DKIM alignment.
DKIM alignment: 99% of observed emails sign with an aligned DKIM key (notification.circle.so, circle.so, email.circle.so); the rest sign with third-party keys (zendesk.com) where DMARC has to rely on SPF alignment.
TLS: 0% TLS 1.3, 100% TLS 1.2 (across 127 sends).
Delivery: fast — 100% of emails in the last 7 days arrive in under 10 seconds (across 37 sends).
Gmail's own filter showed these warnings to real subscribers in their inbox — independent of SPF/DKIM/DMARC pass/fail status. A consistent pattern often signals reputation issues or brand-impersonation concerns.
Brands often use multiple ESPs (e.g. one for marketing, another for transactional/notifications).
Zendesk 3 Substack 2ESPs we've seen sending email for this brand. Green = confirmed by SPF, sender IP rDNS, or known infra pairing (label shows the source). Grey = observed but not yet confirmed by SPF or PTR.
ESPs this brand has authorized in their SPF record. We confirm them with a green ✓ only after seeing recurring use (3+ emails across 2+ days). Until then they stay here — either we haven't observed any email yet, or we've seen a handful but not enough to call them a regular sender for the brand.
Probed: circle.so, bf54x.hubspotemail.net, notification.circle.so, email.circle.so, auth.circle.so
Folder distribution across all emails from this brand observed in Gmail inboxes. High inbox % = good deliverability; spam appearances are the headline signal.
| Folder | Count | Share |
|---|---|---|
| inbox | 261 | 99.2% |
| promotions | 2 | 0.8% |
Which ESP's emails are landing where. Sorted by spam % so problem senders surface first. Brands using one ESP for marketing and another for transactional often see very different placement profiles per ESP. Per-ESP folder data only started being captured recently — counts here may be smaller than the total above until backfill catches up.
| ESP | Emails | Placement | Inbox % | Tabs % | Spam % |
|---|---|---|---|---|---|
| Mailgun | 218 | 100.0% | 0.0% | 0.0% | |
| HubSpot | 28 | 92.9% | 7.1% | 0.0% | |
| Zendesk | 3 | 100.0% | 0.0% | 0.0% | |
| Substack | 2 | 100.0% | 0.0% | 0.0% |
Daily observations per ESP. Brands often run multiple ESPs concurrently — marketing on one, transactional on another — so this chart shows usage rather than switches.
IP addresses circle.so has been observed sending from, and where their mail landed in Gmail.
| IP | Hostname | Obs | Placement | Reputation |
|---|---|---|---|---|
| 198.244.59.202 | c202.c09224bb.usw1.send.mailgun.net |
67 | 6 | Check → |
| 198.244.59.154 | c154.c09224bb.usw1.send.mailgun.net |
59 | 9 | Check → |
| 198.244.59.134 | c134.c09224bb.usw1.send.mailgun.net |
46 | 3 | Check → |
| 198.244.59.200 | c200.c09224bb.usw1.send.mailgun.net |
40 | 6 | Check → |
| 198.244.59.136 | c136.c09224bb.usw1.send.mailgun.net |
40 | 7 | Check → |
| 198.244.59.167 | c167.c09224bb.usw1.send.mailgun.net |
36 | 2 | Check → |
| 198.244.59.105 | c105.c09224bb.usw1.send.mailgun.net |
36 | 3 | Check → |
| 198.244.59.142 | c142.c09224bb.usw1.send.mailgun.net |
28 | 4 | Check → |
| 198.244.59.214 | c214.c09224bb.usw1.send.mailgun.net |
19 | 2 | Check → |
| 143.244.91.109 | bd77gm3.bf54x.hubspotemail.net |
13 | — | Check → |
| 143.244.91.39 | bd77gk5.bf54x.hubspotemail.net |
12 | 3 | Check → |
| 198.244.59.37 | c37.c09224bb.usw1.send.mailgun.net |
10 | 2 | Check → |
| 143.244.91.72 | bd77gl2.bf54x.hubspotemail.net |
4 | 1 | Check → |
| 192.161.149.33 | mta-out3.pod23.use1.zdsys.com |
3 | — | Check → |
| 108.179.150.131 | 4f2oz5.bf54x.hubspotemail.net |
1 | — | Check → |
| 143.244.91.80 | bd77gma.bf54x.hubspotemail.net |
1 | — | Check → |
| 143.244.91.63 | bd77glt.bf54x.hubspotemail.net |
1 | — | Check → |
| 143.244.91.100 | bd77gmu.bf54x.hubspotemail.net |
1 | 1 | Check → |