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From 29 October 2026, vendors and senders across sequencers started reporting a bounce code nobody had seen before, 550 5.7.237, on Microsoft tenants. Here is what it did to three fleets we can see into, in three segments: two of our customers’ and our own.
The data behind this study
Every figure on this page is computed from these files. Fieldwork 20 October to 8 November 2026. Licensed CC BY 4.0: reuse with attribution to WarmInboxes.com.
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Segment 1: tenant blocks / licence wipes across 1,500+ customer tenants at 25 users
1.4–2.1%
Segment 1: bounce rate across those tenants, at 76 sends per tenant on an active day
49.0%
Segment 2: of everything the dense tenants sent on Oct 27–30, this much bounced
0.94%
Segment 2: the same tenants' bounce rate on Nov 1–2, once Microsoft lifted the block
0.62% → 1.09%
Segment 3: our own fleet's bounce rate, Oct 26–30 against Nov 2–5
2.0×
Segment 3: bounce rate on our 100-inbox domains against our 50-inbox domains
Segment 1 came through untouched. Customer tenants at 25 users, sending around 76 a day and up to 162 on the busiest, had no blocks and no licence wipes.
Segment 2 got blocked, then came back. 142 dense customer tenants at 50 or 100 users had their outbound refused with 550 5.7.708 from 20 October 2026. On Oct 27–30, 49.0% of everything they sent bounced. By Nov 1–2 it was 0.94%, and from Nov 3 there wasn’t a single 550 5.7.708. Not one licence was suspended.
Segment 3, our own fleet, stepped up and stayed low. Bounce rate went from 0.62% to 1.09% across 12,103 Microsoft inboxes, about 1.7× and still around one in a hundred, with the densest domains carrying most of it.
| Segment | Tenants | Window | What happened |
|---|---|---|---|
| Segment 1 Customer tenants, 25 users each | 1,500+ | Last 21 days | 1.4–2.1% bounces, no blocks, no licence wipes |
| Segment 2 Customer tenants, 50 or 100 users each | 142 | Oct 20 – Nov 8 | Outbound refused with 550 5.7.708; 49.0% bounced, then 0.94% |
| Segment 3 Our own agency fleet, two workspaces | 12,103 inboxes | Oct 26 – Nov 5 | 0.62% → 1.09%, heaviest on 100-inbox domains |
From 29 October 2026, reports came in from several infrastructure vendors and from senders themselves: Microsoft was restricting Azure tenants used for cold email, and bounces carrying 550 5.7.237 were appearing across sequencers. Operators started calling it a crackdown.
Microsoft has not said what 5.7.237 means. We found no Microsoft documentation for the code as of 6 November 2026. Microsoft’s guidance groups the 5.7.x family as policy and permission rejections, so it reads as a policy refusal rather than a bad address, but that is a reading of the family, not of the code. Paste one into the bounce analyzer and it is named for what it is.
The worst-affected accounts saw something harsher: 550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP. That is not a refusal of one message but of the tenant’s outbound traffic. Microsoft’s own Q&A threads tie it to Exchange Online blocking outbound mail from a low-reputation IP, most often on new or trial tenants. And it started earlier than anyone reported: Segment 2’s Exchange records show 550 5.7.708 from 20 October 2026, nine days before the first 5.7.237 reports.
Working through the affected accounts, the restriction appears to fall on tenants younger than about 10 to 14 days. Domains placed on older tenants performed normally, even sending through the same sequencer IPs as the ones being refused, which points at the tenant rather than the IP pool behind the sequencer.
It fits the three segments. Segment 1’s 1,500+ tenants with no blocks were established tenants, and the prewarmed ones with a month or more of warmup behind them were not affected at all. Segment 2’s refusals ran for about two weeks, from 20 October 2026 to 2 November, and then stopped while the tenants kept sending, which is what tenants ageing out of a restriction would look like. It also fits 550 5.7.708’s association with new tenants.
This is a diagnosis from the accounts we worked, not a counted split: the Exchange records do not carry tenant age, and we have not yet tallied every tenant by age against its outcome. We will publish that if the pattern holds.
What it changes for anyone buying Microsoft infrastructure: a brand-new tenant is now the riskiest place to start cold volume. Inboxes on tenants that are already weeks old and warmed, which is what a prewarmed pool is, and sending domains aged before they carry cold mail, are worth more now than they were a month ago.
| Segment 1 | 1,500+ customer tenants WarmInboxes built at 25 users each. Read from the tenants over the last 21 days: tenant count, average and peak daily sends, bounce range, tenant blocks and licence wipes. The fleet's total sends and bounces are worked out from those, with the formula shown. |
| Segment 2 | 142 customer tenants at 50 or 100 users (73 and 69), the ones that were hitting 550 5.7.708 when we pulled the October data. Read from Microsoft's Exchange records: every failed send on all 142 for Oct 20–30, two windows with full send counts (Oct 27–30 and Nov 1–2), hourly checks of Microsoft's bounce notices to Nov 8, and one licence check on 3 November 2026. These overlap in time, so we never add their counts together. Dates as the traces recorded them (India Standard Time). |
| Segment 3 | Our own agency fleet, 12,103 Microsoft inboxes across two workspaces. Sends and bounces per day, per workspace and per inboxes-per-domain configuration, read from the sequencer for 26 October to 5 November 2026. |
| Tenant block | Outbound sending refused for the whole tenant, every user at once: 550 5.7.708. |
| Licence wipe | The tenant's mailbox licences removed, so the users stop existing as mailboxes. |
| Named | No customer, vendor, domain or sequencer. Workspaces are lettered. |
Segment 1
Over the last 21 days, across 1,500+ Microsoft tenants we built at 25 users per tenant and domain, warming ones included:
The numbers
Over 26 October to 6 November 2026, 12 days, that fleet sent up to 1,368,000 emails (1,500 tenants × 76 sends × 12 days), of which 19,152 to 28,728 bounced at the measured 1.4–2.1%. The sends take every tenant at its active-day average every day, so they are the ceiling, and 1,500 is the floor of the tenant count.
Segment 2
142 dense customer tenants, 73 at 50 users and 69 at 100, each sending around 220 cold emails a day. These are the tenants that were getting hit when we pulled the October data, so this shows how hard it hit and for how long, not how many dense tenants got hit.
Between Oct 20 and Oct 30, Microsoft’s records for these 142 tenants show 117,605 failed sends. 113,244 of them, 96.3%, were 550 5.7.708: the tenant’s outbound refused, not a bad address. Every single tenant had them. Oct 20 is as far back as the records go, so it may have started earlier.
| Date | Failed sends | With 550 5.7.708 |
|---|---|---|
| Oct 20 | 13,790 | 13,751 |
| Oct 21 | 9,514 | 9,432 |
| Oct 22 | 10,828 | 10,710 |
| Oct 23 | 17,249 | 17,095 |
| Oct 24 | 11,590 | 11,438 |
| Oct 25 | 14,407 | 14,229 |
| Oct 26 | 9,946 | 9,682 |
| Oct 27 | 4,786 | 4,126 |
| Oct 28 | 10,761 | 9,167 |
| Oct 29 | 14,288 | 13,170 |
| Oct 30 | 446 | 444 |
These records count failures, not sends, so they don’t give a bounce rate on their own. The two windows below do. Oct 30 is a part day.
| Window | Tenants | Sent | Bounced | Rate | 550 5.7.708 |
|---|---|---|---|---|---|
| Oct 27–30 | 59 | 55,191 | 27,067 | 49.0% | 26,750 |
| Nov 1–2 | 135 | 170,290 | 1,595 | 0.94% | 966 |
Oct 27–30: 27,067 of 55,191 bounced. That’s 49.0%, and 98.8% of those were 550 5.7.708. The median tenant bounced 52.0%, 42 of 59 were above 40%, and not one was under 2%.
Nov 1–2: 1,595 of 170,290, or 0.94%, with tenants sending around 600 a day. The median tenant bounced 0.3%, 132 of 135 were under 2%, and there was no 5.7.237 on any of them.
The two windows don’t cover exactly the same tenants, so we checked the 52 that are in both: 48.5% on Oct 27–30, 1.72% on Nov 1–2. Same tenants, same drop.
Inbox count didn’t make a difference here. On Oct 27–30 the 50-inbox tenants bounced 49.9% and the 100-inbox tenants 46.5%, even though the 50s were sending more each (around 465 a day against 144).
We check Microsoft’s bounce notices every hour. This is how many tenants showed any 550 5.7.708 each day, out of the ones we could read in full.
| Date | Tenants with 550 5.7.708 | Tenants checked |
|---|---|---|
| Oct 31 | 24 | 126 |
| Nov 1 | 3 | 125 |
| Nov 2 | 1 | 125 |
| Nov 3 | 0 | 125 |
| Nov 4 | 0 | 125 |
| Nov 5 | 0 | 125 |
| Nov 6 | 0 | 125 |
| Nov 7 | 0 | 125 |
| Nov 8 | 0 | 125 |
No licence was suspended. When we checked on 3 November 2026, 135 tenants had their Microsoft licences enabled and none were suspended; the other 7 couldn’t be read that day. 550 5.7.708 blocks a tenant’s mail. It doesn’t take its mailboxes away.
Segment 3
12,103 Microsoft inboxes across two workspaces, 26 October to 5 November 2026: 188 bounces on 19,876 sends, 0.95%.
| Date | Sent | Bounced | Rate |
|---|---|---|---|
| Oct 26 | 1,587 | 6 | 0.38% |
| Oct 27 | 496 | 4 | 0.81% |
| Oct 28 | 2,711 | 21 | 0.77% |
| Oct 29 | 1,369 | 8 | 0.58% |
| Oct 30 | 399 | 2 | 0.50% |
| Oct 31 | 30 | 0 | 0.00% |
| Nov 1 | 83 | 3 | 3.61% |
| Nov 2 | 4,976 | 59 | 1.19% |
| Nov 3 | 3,338 | 36 | 1.08% |
| Nov 4 | 2,491 | 22 | 0.88% |
| Nov 5 | 2,396 | 27 | 1.13% |
The step came on Nov 2. Oct 26–30 ran at 0.62% (41 on 6,562); Nov 2–5 at 1.09% (144 on 13,201). Nov 2 was also the biggest send day by far, 4,976, so volume and the new code arrived together. Oct 31 and Nov 1 sent 113 between them; Nov 1’s 3.61% is 3 bounces and reads as noise.
| Configuration | Sent | Bounced | Rate | Share of bounces |
|---|---|---|---|---|
| 100 inboxes per domain | 14,120 | 152 | 1.08% | 80.9% |
| 50 inboxes per domain | 4,197 | 23 | 0.55% | 12.2% |
| 25 inboxes per domain | 1,548 | 13 | 0.84% | 6.9% |
| 3 or 1 inbox per domain | 11 | 0 | 0.00% | 0.0% |
The 100-inbox domains carry 80.9% of the bounces. They carry most of the volume too, but they still bounce at 2.0× the rate of the 50-inbox domains, 1.08% against 0.55%.
It is not a clean line: the 25-inbox domains sit between them at 0.84%, on 1,548 sends, which is too few to rank against the 50s with confidence. Read it with segments 1 and 2, where the difference between 25 and 50+ users was blocks against none.
| Workspace | Sent | Bounced | Rate | Share of bounces |
|---|---|---|---|---|
| Workspace A | 12,240 | 133 | 1.09% | 70.7% |
| Workspace B | 7,636 | 55 | 0.72% | 29.3% |
Hit by this?
If your tenants are bouncing 550 5.7.708 or 5.7.237, you don’t have to rebuild on your own. Tell us what you pay your current vendor for fresh Azure inboxes and, depending on your volume, we’ll match it.
At that price you get the whole suite:
And you get the team that ran Segment 1 through this. We set tenants up at 25 users, keep each one’s daily sends in the range that came through untouched, and warm new tenants before they carry cold mail, because a brand-new tenant is exactly what got hit. If you need to send this week, our prewarmed Azure tenants are already past that age (prewarmed isn’t part of the price match).
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