Managing Mailbox Quota

Important Note

The Exchange Online Archiving feature will be enabled for all users following their migration. This can be utilised as an alternative option to procuring top-up licenses if existing quotas have been met or exceeded.

Managing mailbox quotas in Exchange Online

NHSmail Refresh mailbox sizing process:

As part of the national licencing agreement and NHSmail Refresh, the management of mailboxes will change.

The mailbox size in Exchange Online is determined by the current on-premise mailbox storage utilisation:

  • Users with existing on-premise mailbox utilisation larger than 4GB (excluding utilisation of the Exchange dumpster feature) will automatically be assigned 50GB mailboxes during the migration to Exchange Online
  • Users with existing on-premise mailbox utilisation smaller than 4GB will automatically be assigned 4GB mailboxes during the migration to Exchange Online
Important Note

The above logic also applies to organisations that have previously had top-up agreements for larger mailboxes. For example, a user that currently has a 25GB mailbox – but only utilises 3GB, will get a 4GB mailbox during the Refresh process. The existing top-up process is deprecated in Exchange Online, guidance is included below on how organisations can look to access larger mailbox quotas.


All Shared and Resource mailboxes will receive a 50GB allocation as standard following the NHSmail Refresh. Please also note that shared mailboxes cannot be reduced back to 4GB.


Mailbox quotas in Exchange Online:

Each organisation can permit up to 10% of their Exchange Online users to increase their allocated mailbox size from 4GB to 50GB, at no additional cost. The managing your organisation’s 10% quota section below details how LAs can allocate 50GB mailboxes to users.

The 10% quota is determined against organisation ODS code and is calculated based on the number of users in Exchange Online. Therefore, each organisations quota will change as more users are migrated as part of the Refresh – the quota is recalculated daily, so if more users get licensed through the day, they would not be included in the quota calculation until the next day. If the full 10 % was initially allocated for an organisation, and the number of users then drops, the users will keep the larger mailboxes and the organisation won’t be able to assign anymore.

It is possible that through migration that the 10% quota is exceeded. This will not prevent mailboxes being added with 50GB at the migration stage, but at the end of migration, organisations will be expected to ensure they are within the specified 10% quota.

If you require additional large mailboxes in excess of your 10% quota, you will need to procure O365 licences (outside of the nationally provided license) and onboard them to the NHSmail central tenant. Please follow the onboarding for Local Administrators guidance in order to do so. Locally procured licenses will be in addition to the 10% national allocation.

Managing mailbox quotas through the NHSmail portal

Mailbox size management varies based on whether you are managing the 10% quota made available through the national deal, or if your organisation has procured O365 licenses directly.

Managing your Organisation’s 10% quota:

As highlighted in the section above, users will be allocated 4GB or 50GB mailboxes based on their current on-premise mailbox utilisation. As part of the Refresh, users automatically get put into their national user policy – more information can be found in the Introduction and creating a new policy guidance.

Important Note

National user policies can include users with both 4GB and 50GB mailboxes in Exchange Online.

Should you wish to increase a user’s mailbox to 50GB, please follow the below steps:

1. On the NHSmail Portal go to the User Details page via the Admin section.

2. Click on Edit Subscriptions within the Service Properties section.


Useful Information

  • If you are not able to click on the Edit Subscriptions button, you may not have the right permissions to manage mailbox sizes. You must have both the LA and Authorisation role to manage mailbox sizes.  Currently only LAs with the Authorisation role can manage mailbox size for users, both on premise and in Exchange Online.
  • If you are an LA allocating a larger mailbox on behalf of another organisation you need to ensure that you have the Authorisation role for that organisation. Please see information on how to add an Authorisation role. 

  • You can view the number of larger mailboxes set for the organisation in the Organisation Export Report under the Organisations tab. Please see the screenshot below for more information.


  1. On the Mailbox size, select the desired mailbox size and confirm.

 1. If under the 10% quota, the user will be assigned the larger mailbox once confirmed. It may take a small amount of time before this becomes visible to the user.

2. If the user’s organisation has reached the quota for larger mailboxes, you will not be able to assign a larger mailbox to the user. The number of assigned large mailboxes will need to be reduced to below the quota before a large mailbox can be assigned.

Managing quotas for directly procured O365 licenses:

Quota management for licenses procured directly by an organisation is handled through user policies in the NHSmail Portal.

Once you have onboarded your license subscription, please follow the below steps to configure a user policy with large mailboxes enabled.

  1. On the NHSmail Portal go to the User Policy Management page via the Admin section.
  2. When configuring a locally procured license, select the correct subscription from the drop-down menu – for example E1, E3 or E5.
  3. Exchange Online users will either have a 4GB or 50GB mailbox depending on their mailbox size prior to the migration. A Local Administrator (LA) has the option to provision a larger mailbox by toggling the Large Mailbox Size Enabled option to On.

4. All users within the policy will then be given the standard mailbox size attributed to the O365 license type, as shown in the table below.


E1 50GB
E3 100GB
E5 100GB
Important Note

Configuring large mailbox policies is only possible for users that reside in Exchange Online. Adding on-premise users to a policy with the large mailbox toggle enabled will not take effect, their current on-premise mailbox quota will remain as-is.

Organisations with ‘hybrid’ licenses:

For organisations that have already procured and are using O365 licenses on-premise, as part of the previously known Hybrid service – there are some additional steps to be aware of.

Users allocated locally procured O365 licenses will not be moved into your organisations national user policy upon migration, instead they will be added into a ‘larger mailbox’ policy automatically. LAs will start to see this type of policy appear as the migration progresses.

This is to ensure users can access the default Microsoft storage available from the point of migration. LAs can manage users included within the ‘larger mailbox’ policy, as per the user policy management guidance.

Granting Authorisation role

  • Primary Local Administrator(s) can assign Authorisations role to their own account.
  • Local Administrator(s) can assign Authorisations role to their own account.
  • Please see Adding Administration Roles on how to assign a role
Updated on 08/03/2022

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