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Do not treat Microsoft 365 shared mailboxes, distribution lists, dynamic distribution groups, and Microsoft 365 Groups as interchangeable. Proton documents Groups as a distribution-list-style feature, while its Microsoft 365 Easy Switch guide says shared mailboxes are not supported in that user-account migration flow. Before switching, identify what each object does, map its permissions and dependencies, and pilot the replacement. That limitation describes Proton’s documented migration path; it does not establish that no manual workaround exists.
What Microsoft 365 object do you actually have?
Start with the object’s job, not just its email address. Microsoft distinguishes shared mailboxes, distribution lists, dynamic distribution groups, and Microsoft 365 Groups because they behave differently. Use Microsoft’s group comparison and group management documentation to confirm the type and configuration in your tenant.
| Microsoft 365 object | What it is for | What to check before choosing a Proton approach |
|---|---|---|
| Shared mailbox | A mailbox multiple people access, potentially with a collaborative calendar. | Who needs to read, send from, or send on behalf of the address; historical mail and sent-copy expectations; calendar use; and how the team will share access after migration. |
| Standard distribution list | An email address that distributes messages to a defined membership list. | Who may send to the list, whether external senders or members are involved, and whether the membership is manually maintained. |
| Dynamic distribution group | A recipient list calculated from rules when a message is sent. | The membership rules and whether they can be reproduced or should instead become a maintained static list. Proton’s Groups documentation describes forwarding to group members; it does not establish a matching rule-based membership feature. |
| Microsoft 365 Group | A broader collaboration space that can include a group mailbox and calendar, plus connected services such as Planner and SharePoint depending on subscription and setup. | Every mailbox, calendar, file, task, and collaboration dependency—not just the group email address—and where each will live after the change. |
Microsoft’s current overview states that a Microsoft 365 Group can have more than 1,000 members, but only 1,000 can access group conversations concurrently; it also lists a 50 GB group mailbox. These are Microsoft-published product limits, not assurances about migration or Proton capacity. See the Microsoft 365 Groups overview.
Does Proton have shared mailboxes?
Proton’s Microsoft 365 business Easy Switch guide explicitly says: “Microsoft 365 shared mailboxes are not currently supported and will not appear in this list.” The guide covers migration of user accounts, with selectable emails, contacts, and calendars; it does not document shared-mailbox migration through that flow. Read the current Proton Easy Switch for Microsoft 365 guide before planning a cutover.
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Map Microsoft shared mailbox permissions separately
Microsoft treats Full Access, Send As, and Send on Behalf as distinct permissions. Full Access lets a user open and manage mailbox contents; Send As permits sending as the shared address; Send on Behalf identifies the sender as acting on behalf of it. A new shared mailbox has sign-in blocked by default. Check the current assignments and expected behavior rather than assuming one permission implies another. Microsoft documents setup and behavior in its shared mailbox setup guide.
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Also record whether the mailbox is hidden from address lists. Microsoft notes that visibility can affect Send As or Send on Behalf behavior in Outlook desktop, so validate the actual client workflow if the mailbox is hidden.
How do Proton Groups compare to Microsoft 365 groups?
Proton Groups are closest to an email distribution-list use case: mail sent to a group address is forwarded to its members. Proton describes this as commonly known as a mailing list or distribution list in its Groups documentation. That is not evidence of equivalence with a Microsoft 365 Group’s shared mailbox, calendar, or connected collaboration resources.
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Proton documents sender options of Everyone, Group members, and No one, with per-member overrides. Test the required sender rules and membership behavior, including invitation acceptance and external participants. Proton also warns: “If you add a non-Proton Mail email address to the group, end-to-end encryption for messages going to the group address will be turned off.” If external addresses are necessary, decide whether that encryption trade-off is acceptable before rebuilding the list.
Plan eligibility can change. Proton’s Groups article lists Mail Professional (now deprecated), Proton Workspace, and Visionary with a custom domain as eligible plans; confirm current plan availability and requirements directly in the documentation before relying on a feature.
What should you inventory before migrating?
For every mailbox, list, and group, capture the current address and aliases, purpose, owners, members, senders, and the rules governing who may send. Include dependencies beyond email and the data the team expects to keep.
- Membership: static recipients, dynamic membership conditions, external members, and the owner responsible for updates.
- Sending and access: who reads messages, who may send to the address, and who sends as or on behalf of it.
- Resources: shared calendars, files, tasks, automations, and collaboration services tied to the object.
- History and governance: historical mail, retention, audit needs, and expectations for shared sent messages.
- Addressing and security: primary address, aliases, external senders, encryption expectations, and any client-specific behavior.
- Migration path: the specific transfer or rebuild method for each object, not a blanket assumption based on its address.
Microsoft’s conversion options are also conditional. Its distribution-list-to-Microsoft-365-Group upgrade applies to eligible cloud-managed, simple, non-nested lists. Microsoft separately documents converting a distribution list into a shared mailbox by freeing the address, creating the mailbox, adding members, and assigning permissions. Microsoft states that a shared mailbox cannot be migrated to a Microsoft 365 Group. These are Microsoft-to-Microsoft procedures, not Proton migration routes: see the distribution-list upgrade guide and conversion to shared mailboxes.
How do you pilot the change safely?
- Export or document the inventory. Record the addresses, aliases, owners, members, permissions, sender restrictions, dynamic rules, dependencies, history needs, and automations for each object.
- Classify each object by job. Mark it as a shared operational inbox, an email-only list, a rule-based distribution group, or a collaboration workspace. Keep shared mailboxes distinct because Proton’s documented Easy Switch flow excludes them.
- Define the shared-mailbox replacement workflow. Specify how incoming messages will reach the team, how replies will use the shared address, where sent copies and old mail will be handled, and how access and audit requirements will be met. Validate those requirements with Proton or the chosen migration provider.
- Rebuild representative email lists. Test a simple list and one involving external senders or members. Verify sender permissions, member invitations, message delivery, and the encryption consequence of adding a non-Proton address.
- Map collaboration resources separately. For each Microsoft 365 Group, decide whether its calendar, files, Planner tasks, and other services can be retired, replaced with separate tools, or must remain available elsewhere.
- Run a representative pilot before changing mail flow. Include a simple list, a list with external participants, a shared mailbox, and a collaboration-heavy Microsoft 365 Group. Use real test users to check client behavior, permissions, data/history handling, and any required automations.
- Plan the Easy Switch operation and cutover. Proton’s guide requires the operator to be both a Proton administrator and a Microsoft 365 administrator, and the Microsoft 365 accounts must be cloud-hosted rather than on-premises. It describes migrating user accounts in batches and a final MX-record change; it also warns that unused activation links stop working after migration is finalized. Confirm operational details in the current guide before scheduling the change.
What does the documentation not settle?
The official documentation establishes the described product behavior and Easy Switch scope, but it does not provide a complete object-by-object mapping for every tenant setting, history requirement, or workflow. In particular, it does not establish that a Proton Group reproduces all Microsoft 365 Group resources or all shared-mailbox delegation behavior. Treat requirements that depend on those details as acceptance tests for the pilot, not assumptions.
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