The three limits that catch small teams are the ones nobody reads until they bite: a Microsoft 365 shared mailbox supports 25 users and stores 50 GB without a licence, and a Google Workspace account can send 2,000 messages a day, dropping to 100 recipients per message when a third-party tool connects over SMTP or IMAP. Everything below is taken from Microsoft’s and Google’s own documentation, with the source for each figure at the end.
Microsoft 365 shared mailbox
| Limit | Documented value |
|---|---|
| Maximum users | 25. Beyond that, Microsoft warns of connection failures or duplicated messages |
| Storage without a licence | 50 GB |
| Storage with Exchange Online Plan 2 | 100 GB |
| Storage, mailbox created before July 2018 | 100 GB, unlicensed |
| Licence for the mailbox itself | None required, up to 50 GB |
| Licence for each person accessing it | A licensed Exchange Online mailbox of their own |
| External access | Not possible. A Gmail account cannot be given access |
| Direct sign-in | Should be blocked, and kept blocked |
| Password | Exists, system-generated, not intended for use |
| Preventing deletions | Not possible in a shared mailbox |
| Encryption of outgoing mail | Not possible: no security context of its own, so no encryption key |
| Migration to a Microsoft 365 group | Not possible |
| Subfolders per folder | 10,000 |
Three of these deserve emphasis, because they are the ones that change a decision rather than a setting.
The 25-user ceiling is not a soft one. Microsoft’s own remedy is to use a Microsoft 365 group instead, and its description of what goes wrong past that point names duplicated messages. A platform that produces duplicates under load is a poor foundation for a team already fighting duplicate replies by hand.
The storage ceiling has a nasty failure mode. When a shared mailbox fills up, it keeps receiving for a while, then stops sending, then starts bouncing incoming mail back to whoever wrote to you. On a support address running for several years with attachments, 50 GB arrives sooner than teams expect, and the first symptom your customers see is a bounce.
The migration path is one-way. You can convert a user mailbox into a shared mailbox. You cannot convert a shared mailbox into a Microsoft 365 group. Choosing between them is a decision to make deliberately at the start, covered in shared mailbox vs distribution list.
Gmail and Google Workspace
Google publishes no equivalent of a shared mailbox, so the relevant limits are those of delegation and of sending. The sending ones apply to the account behind your shared address, whoever is writing.
| Limit | Documented value |
|---|---|
| Delegates, work or school account | Up to 1,000 configured |
| Delegates connected at the same time | About 40 with typical use |
| Delegates, personal @gmail.com account | 10 |
| Messages sent per day, per user | 2,000 (1,500 for mail merge, 500 on a trial account) |
| Recipients per message | 2,000 total, of which 500 external maximum |
| Recipients per message over SMTP, POP or IMAP | 100 |
| Recipients per message through the Gmail API | 500 |
| Total recipients per day | 10,000 |
| External recipients per day | 3,000 |
| Unique recipients per day | 3,000, of which 2,000 external |
| After hitting a limit | No sending for up to 24 hours, receiving continues |
The line to notice is the 100-recipient limit over SMTP, POP and IMAP. It is twenty times lower than the interface limit, and it applies precisely when a third-party tool sends on your behalf over IMAP rather than through the API. If you send a seasonal announcement to your 250 members from a tool connected that way, it will not go out in one message. That is not a bug in the tool.
The daily limits also apply per account, not per person. Three volunteers all working the same delegated mailbox share one 2,000-message allowance, and one of them running a mail merge consumes it for everyone.
Google Groups used as a team inbox
A Google Group with Collaborative Inbox enabled is the closest Google equivalent, and its constraints are functional rather than numeric.
| Constraint | Detail |
|---|---|
| Conversation history | Must be enabled, or Collaborative Inbox features do not work at all |
| Assigning a conversation | Requires the “Who can moderate metadata” permission |
| Marking complete | Requires the “Who can moderate metadata” permission |
| Marking a duplicate | Requires the “Who can moderate content” permission |
| Effect of marking a duplicate | The conversation is locked |
| Deleting a resolved conversation | Permanent, cannot be restored |
| Who enables it | A group owner or manager, and Groups for Business must be on |
The permission model is the practical obstacle. A volunteer who can read and reply but was never granted moderate-metadata rights cannot take a conversation, which means the one feature you enabled the group for is invisible to them. This is a common reason teams conclude Collaborative Inbox “does not work”.
Distribution groups
| Limit | Documented value |
|---|---|
| Maximum message size, group with 5,000 to 99,999 members | 25 MB |
| Maximum message size, group with 100,000 members | 5 MB |
| Central archive of past messages | None |
| Microsoft’s stated purpose | Broadcasting to a set group of people |
What none of them limit, and should
Read the tables again and notice what is missing. Not one of these products documents a limit on how many people can reply to the same message at the same time, because not one of them tracks it. The ceilings are all about capacity: storage, recipients, users, message size. The failure that actually costs a small team money is coordination, and it has no quota because it is not a resource anyone is metering.
That is the real gap. A shared mailbox at 24 users and 40 GB is inside every documented limit and can still send a customer two contradictory quotes on the same morning, which is what we described in how to avoid duplicate replies.
Where Trupeo sits against these
Trupeo connects on top of the mailbox you already have, so your provider’s limits still apply: the storage is theirs, the sending quotas are theirs, and your address does not change. What it removes is the ceiling that belongs to the tool rather than the mailbox.
There is no 25-user ceiling, because Trupeo is not several people signing into one account: each person has their own login against a mailbox connected once. The pricing is flat rather than per user, so the number of people is not a cost decision either. And the coordination layer that none of the tables above contains, one owner per conversation, an open or done status, internal notes and a signal when a colleague is already replying, is the part you were missing.
See our pricing and the features, or read the complete shared inbox guide for the whole subject.
Frequently asked questions
How many users can access a Microsoft 365 shared mailbox?
Microsoft documents a maximum of 25. Past that, people accessing it at the same time might experience connection failures or duplicated messages, and Microsoft’s recommendation is to use a Microsoft 365 group instead.
What is the storage limit of a shared mailbox?
50 GB without any licence assigned to it, and 100 GB if you assign an Exchange Online Plan 2 licence. Unlicensed shared mailboxes created before July 2018 have 100 GB. When it fills, the mailbox keeps receiving for a while, then stops sending, then bounces incoming mail.
How many emails can a Google Workspace account send per day?
2,000 messages per day per user, or 1,500 when using mail merge and 500 on a trial account. The daily recipient total is 10,000, of which 3,000 may be external. After hitting a limit, sending stops for up to 24 hours while receiving continues.
Why can I only send to 100 recipients through IMAP?
That is Google’s documented limit for SMTP, POP and IMAP, against 2,000 in the Gmail interface and 500 through the Gmail API. It applies whenever a tool sends on your behalf over those protocols, which catches teams sending a seasonal announcement from a third-party client.
Does a shared mailbox need its own licence?
No, up to 50 GB. But every person who accesses it needs their own licensed Exchange Online mailbox, so the shared mailbox is free only in the sense of adding nothing to subscriptions you already pay.
Can people outside my organisation use our shared mailbox?
No. Microsoft states you cannot give people outside your business, such as people with a Gmail account, access to a shared mailbox, and suggests a Microsoft 365 group when external people must be included. A tool connecting over the provider’s API is the other way round that constraint, which we cover in a shared inbox for Gmail and Outlook.
Sources:
- About shared mailboxes in Microsoft 365: the 25-user maximum and its connection failures or duplicated messages warning, the 50 GB and 100 GB storage limits, the Exchange Online Plan 2 requirement, the 100 GB of mailboxes created before July 2018, the licence required per accessing user, the impossibility of external access, of preventing deletions and of encrypting outgoing mail, and the instruction to block sign-in.
- Exchange Online limits: the shared mailbox, user mailbox and group mailbox storage quotas per plan, the 10,000 subfolders per folder, and the maximum message size for large distribution groups.
- Compare types of groups in Microsoft 365: the impossibility of migrating a shared mailbox to a Microsoft 365 group, and the stated purpose of distribution groups.
- Gmail sending limits in Google Workspace: the 2,000 messages per day, the 2,000 recipients per message with 500 external maximum, the 100 recipients per message over SMTP, POP and IMAP, the 500 through the Gmail API, the 10,000 daily recipients, the 3,000 external and 3,000 unique daily recipients, and the up-to-24-hour sending pause.
- Delegate and collaborate on email: the 1,000 delegates on a work or school account, the 10 on a personal account, and the roughly 40 delegates connected at the same time with typical use.
- Use a group as a Collaborative Inbox: the moderate metadata and moderate content permissions, the locking of a conversation marked as a duplicate, and the permanence of deleting a resolved conversation.
- Make a group a Collaborative Inbox: the requirement to enable conversation history first, and who can turn the features on.