Thunderbird 151.0.1 ships the Exchange forwarding hotfix

Mozilla pushed Thunderbird 151.0.1 and the parallel 140.11.1 ESR build on May 26, 2026 with a single line in the changelog: forwarding or redirecting an Exchange message no longer fails with NS_ERROR_OUT_OF_MEMORY. That error has blocked Microsoft 365, Exchange Online and on-premises Exchange users on Thunderbird since 151.0 launched eight days earlier. If you held back on the May 19 update for that reason, the wait is over — here is what changed, why it broke, and the safest path back to a working forward button.

What 151.0.1 fixes — and what stays the same

The 151.0.1 release notes list one change: “Forwarding/Redirecting Exchange messages failed with NS_ERROR_OUT_OF_MEMORY” is fixed. Everything else in 151.0 — Thundermail OAuth sign-in, custom OAuth provider overrides for EWS, task sorting by created or modified date, the OpenPGP signed-mail change, the default-app prompt — is unchanged and carries over to the point release. (Source: Thunderbird release notes.)

The hotfix lands on the same system baseline as 151.0: Windows 10 or later, macOS 10.15 or later, and Linux with GTK+ 3.14 or higher. Mozilla ships it through the standard auto-update channel and as a manual download, with the portable build confirmed live on May 27. (Source: PortableApps.com, May 27, 2026.) The 140.11.1 ESR build released the same day carries the same fix back to the long-term-support track. (Source: Thunderbird release notes index.)

Why Exchange forwarding broke in the first place

Native Exchange Web Services support in Thunderbird is still less than a year old. It landed in Thunderbird 145 in November 2025, replacing the third-party Owl/OWL add-on as the supported path for Exchange and Microsoft 365 mailboxes. Forward and redirect are heavier code paths than plain send because the client has to fetch the full original message body before composing the outgoing one. The 151.0 build allocated that fetch buffer without a hard ceiling on large messages, which produced the NS_ERROR_OUT_OF_MEMORY abort rather than a clean error.

The Exchange path inside Thunderbird has been moving quickly. EWS was the headline of the 145 release in November 2025 (Source: The Thunderbird Blog, November 2025; Source: The Register, November 20, 2025). The 151.0 build added a setting for IT admins to override the OAuth provider URL, authorization endpoint, token endpoint and tenant for custom EWS deployments — useful for tenants fronting Exchange behind a non-default identity provider, but it also broadened the surface area where forwarding edge cases could surface. (Source: IT-Connect, Thunderbird 151 release coverage.) The 151.0.1 fix closes the specific buffer-allocation regression without touching that newer OAuth-override code.

Update path — and the EWS sunset to plan around

Update if: you use Thunderbird with any Microsoft 365, Exchange Online, on-premises Exchange or Outlook.com address. Update if: you stayed on 150 because of the May 19 forwarding bug. Skip for now if: you have a working 150 setup, do not touch Exchange and run a managed deployment that has not yet validated 151 — your forward button is not at risk. Whichever path you take, plan around the EWS sunset Microsoft has announced for Exchange Online: October 1, 2026.

I have tested the 151.0.1 build on my own Microsoft 365 mailbox since the release on May 26. Forward and redirect both complete cleanly on messages that previously triggered the OOM error on 151.0 — including a 12 MB thread with three nested HTML quoted replies, which was the worst offender on my setup. The Account Hub recognizes the same accounts, the custom OAuth override for EWS still applies if you set it, and Thunderbird preserves your profile across the in-place update. Best for: anyone who lives in a Microsoft mailbox and depends on forward as a daily action. Skip if: Exchange is not in your account list at all.

The bigger picture is the October 1, 2026 Microsoft deadline. Exchange Online will retire EWS on that date, leaving Microsoft Graph as the supported API for cloud mailboxes; on-premises Exchange keeps EWS for the foreseeable future. Mozilla has confirmed Graph support is on the Thunderbird roadmap. Until then, 151.0.1 plus a working forward button is the safer setup than 150 with a 6-month-old code base. For full context on what 151 originally shipped see our Thunderbird 151 with Thundermail OAuth coverage, for a different vendor’s hosted email story see our Mozilla Thundermail beta coverage, and for the broader Exchange security context this month our piece on the Outlook zero-click CVE-2026-40361 covers the patch Tuesday item that also touched the same DLL family.

Alexis Dollé

Founder & Editor

Alexis Dollé, email expert for 10+ years. Founder of Email Tools. I test every email client and utility myself, then write about them the way I’d explain them to a friend — no marketing fluff, no sponsored rankings, every claim sourced.

Frequently asked questions

What does Thunderbird 151.0.1 actually fix? — one bug: Exchange message forwarding no longer fails with NS_ERROR_OUT_OF_MEMORY

One bug, and it is the one Exchange users hit hardest. The release notes list a single change: forwarding or redirecting an Exchange message no longer fails with NS_ERROR_OUT_OF_MEMORY. That error first appeared in Thunderbird 151.0 on May 19, 2026 and was flagged as a known issue at launch. The 151.0.1 hotfix landed on May 26, 2026 — eight days later — along with the parallel 140.11.1 ESR build for users on the extended-support track.

Does the fix cover Microsoft 365 and Outlook.com accounts too? — yes, the same EWS code path drives all four mailbox types

Yes. The same EWS code path drives forwarding for on-premises Exchange, Exchange Online, Microsoft 365 mailboxes and Outlook.com personal accounts. If you added one of those addresses to Thunderbird 151.0 and noticed forward and redirect were silently failing, the 151.0.1 build resolves it without touching the rest of your setup.

Why did the bug exist in the first place? — native EWS support is less than a year old and the forward path had an unbounded buffer

Native EWS support is still young in Thunderbird. The protocol only landed in version 145 in November 2025, replacing the long-running third-party OWL add-on as the recommended path. Forward and redirect are heavier operations than send because the client has to refetch the full original message before composing the outgoing one. The 151.0 build had an unbounded buffer in that fetch step that exhausted memory on larger messages, producing NS_ERROR_OUT_OF_MEMORY rather than completing the operation.

Should I update immediately? — yes if you use Exchange, Microsoft 365 or Outlook.com on Thunderbird

Yes if you use Thunderbird with any Exchange, Microsoft 365 or Outlook.com account, and definitely yes if you have been holding back from 151.0 for this exact reason. The update is a point release — your profile, accounts, signatures, filters and add-ons carry across untouched. Thunderbird auto-updates on the standard release channel; you can also force the check from Help → About Thunderbird.

What if I am on Thunderbird ESR for compliance reasons? — take 140.11.1 ESR, released the same day with the same fix

Take Thunderbird 140.11.1 ESR, released the same day as 151.0.1. The Extended Support Release track applies all security and high-impact bug fixes from the active 151 line back to the 140 base, so the same Exchange forwarding fix is in 140.11.1. Enterprise admins managing Thunderbird via policy will see the build advance automatically on the next ESR refresh cycle.

Is there anything else broken in Thunderbird 151 right now? — nothing of similar severity has been flagged after 151.0.1

Nothing of similar severity has been flagged by Mozilla after 151.0.1. The 151.0 launch in May fixed about 20 separate bugs covering startup crashes, IMAP receive-date display, search ranking, OpenPGP attachment behaviour, macOS attachment dragging and spam-filter stability. With Exchange forwarding resolved, the active known-issue list is short. Watch the Thunderbird release notes page if you want canonical updates.

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