{"id":5418,"date":"2026-08-27T15:22:29","date_gmt":"2026-08-27T15:22:29","guid":{"rendered":"https:\/\/geekmungus.co.uk\/?p=5418"},"modified":"2026-08-27T15:22:29","modified_gmt":"2026-08-27T15:22:29","slug":"microsoft-exchange-online-m365-maximum-email-message-size-limits-send-and-receive","status":"publish","type":"post","link":"https:\/\/geekmungus.co.uk\/?p=5418","title":{"rendered":"Microsoft Exchange Online (M365) Maximum Email Message Size Limits (Send and Receive)"},"content":{"rendered":"\n

Microsoft has announced<\/a> that Office 365<\/a> (Exchange Online) now supports up to 150MB email messages, quite a large increase from the previous limit of 25MB. Within Exchange Online you don’t get the level of control you had on Microsoft Exchange on-premise servers, where you could configure the size limits on Organisation Wide transport configuration, Send\/receive connectors, AD site links or individual recipients.<\/p>\n\n\n\n

We have a standard configuration setting for maximum email message size across all components of the email infrastructure to avoid confusion and unexpected behaviour.<\/p>\n\n\n\n

Note that there was some “fun” in relation to this in August 2026:\u00a0https:\/\/www.reddit.com\/r\/sysadmin\/comments\/1vybq8t\/exo_new_mailboxes_provisioned_with_150_kb_and\/<\/a>, where these issues: EX1454589 and EX1454202 affected provisioning of new mailboxes during a window of time, existing mailboxes did not have their limts changed, the mechanisms at work are decsribed below.<\/p>\n\n\n\n

Organisation Wide Transport Limit<\/h1>\n\n\n\n

Microsoft have configured this to Unlimited, and we can’t change this, in effect this means their maximum supported, i.e. 150MB. All other changes are to be made on the mailboxes themselves, and that is where Mailbox Plans come in.<\/p>\n\n\n\n

PS U:\\> Get-TransportConfig | fl maxreceivesize,maxsendsize\n\nMaxReceiveSize : Unlimited\nMaxSendSize    : Unlimited<\/code><\/pre>\n\n\n\n
Mailbox Plan to Configure Default Email Maximum Message Size Limit<\/h1>\n\n\n\n
The settings configured on a Mailbox Plan are used when a new mailbox is created, its kind of like a template. We need to first ensure all of these are configured to our desired value first, so any new mailboxes will get this setting.<\/p>\n\n\n\n
PS U:\\> Get-MailboxPlan | fl name,maxsendsize,maxreceivesize,isdefault\n\nName           : ExchangeOnline-bb0dc167-36ea-47e4-1234-3705f30c6d4d\nMaxSendSize    : 35 MB (36,700,160 bytes)\nMaxReceiveSize : 36 MB (37,748,736 bytes)\nIsDefault      : False\n\nName           : ExchangeOnlineDeskless-3a21d409-ab6e-2134-baec-0a1a0e93a608\nMaxSendSize    : 35 MB (36,700,160 bytes)\nMaxReceiveSize : 36 MB (37,748,736 bytes)\nIsDefault      : False\n\nName           : ExchangeOnlineEnterprise-cd77a389-95f8-1234-a2d4-aaa606ecff9a\nMaxSendSize    : 35 MB (36,700,160 bytes)\nMaxReceiveSize : 36 MB (37,748,736 bytes)\nIsDefault      : True\n\nName           : ExchangeOnlineEssentials-2ef83bda-7171-1234-a823-5fddc3b0ba7a\nMaxSendSize    : 35 MB (36,700,160 bytes)\nMaxReceiveSize : 36 MB (37,748,736 bytes)\nIsDefault      : False<\/code><\/pre>\n\n\n\n
These are all set to 35MB, we don’t want that.<\/p>\n\n\n\n
Configure Maximum Send and Receive Size Limit<\/h2>\n\n\n\n
So to update each of the Mailbox Plans to say 60MB, we would do the following:<\/p>\n\n\n\n
Set-MailboxPlan ExchangeOnline-bb0dc167-36ea-47e4-1234-3705f30c6d4d -MaxSendSize 60MB -MaxReceiveSize 60MB\nSet-MailboxPlan ExchangeOnlineDeskless-3a21d409-ab6e-1234-baec-0a1a0e93a608 -MaxSendSize 60MB -MaxReceiveSize 60MB\nSet-MailboxPlan ExchangeOnlineEnterprise-cd77a389-95f8-43af-1234-aaa606ecff9a -MaxSendSize 60MB -MaxReceiveSize 60MB\nSet-MailboxPlan ExchangeOnlineEssentials-2ef83bda-7171-4bd5-1234-5fddc3b0ba7a -MaxSendSize 60MB -MaxReceiveSize 60MB<\/code><\/pre>\n\n\n\n
Then check again with:<\/p>\n\n\n\n
PS U:\\> Get-MailboxPlan | fl name,maxsendsize,maxreceivesize,isdefault\n\n\nName           : ExchangeOnline-bb0dc167-36ea-47e4-1234-3705f30c6d4d\nMaxSendSize    : 60 MB (62,914,560 bytes)\nMaxReceiveSize : 60 MB (62,914,560 bytes)\nIsDefault      : False\n\nName           : ExchangeOnlineDeskless-3a21d409-ab6e-45b9-1234-0a1a0e93a608\nMaxSendSize    : 60 MB (62,914,560 bytes)\nMaxReceiveSize : 60 MB (62,914,560 bytes)\nIsDefault      : False\n\nName           : ExchangeOnlineEnterprise-cd77a389-95f8-43af-1234-aaa606ecff9a\nMaxSendSize    : 60 MB (62,914,560 bytes)\nMaxReceiveSize : 60 MB (62,914,560 bytes)\nIsDefault      : True\n\nName           : ExchangeOnlineEssentials-2ef83bda-7171-4bd5-1234-5fddc3b0ba7a\nMaxSendSize    : 60 MB (62,914,560 bytes)\nMaxReceiveSize : 60 MB (62,914,560 bytes)\nIsDefault      : False<\/code><\/pre>\n\n\n\n
Good job.<\/p>\n\n\n\n
Update Exchange On-Premise Send Connector<\/h2>\n\n\n\n
The configuration for the on-premise Send Connector (going to M365) is just a case of updating the maximum value here:<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
You do not need to update the Microsoft Exchange Online end, infact you can’t, you do not have the authoritar, Microsoft control this.<\/p>\n\n\n\n
Check and Remediate (if required) Existing Mailboxes<\/h1>\n\n\n\n
Although I\u2019ve updated the mailbox plan above, existing mailboxes still have the previous settings applied. A change to the mailbox plan has no impact on existing mailboxes. <\/p>\n\n\n\n
Check Individual Mailbox<\/h2>\n\n\n\n
To check an individual mailbox:<\/p>\n\n\n\n
Get-Mailbox Bob.Smith | fl mailboxplan,maxsendsize,maxreceivesize<\/code><\/pre>\n\n\n\n
And we can see the below, it just so happens that because this is an existing mailbox, the settings change to the Mailbox Plan has not changed anything.<\/p>\n\n\n\n
PS U:\\> Get-mailbox \"Bob Smith\" | fl mailboxplan,maxsendsize,maxreceivesize\n\nMailboxPlan    : ExchangeOnlineEnterprise-cd77a389-95f8-43af-1234-aaa606ecff9a\nMaxSendSize    : 58.59 MB (61,440,000 bytes)\nMaxReceiveSize : 58.59 MB (61,440,000 bytes)<\/code><\/pre>\n\n\n\n
Check All Mailboxes<\/h2>\n\n\n\n
Unless you are a mentalist, you’ll want this to be exported to a CSV. The below exports all the user mailboxes to a CSV with the settings. You may want to add some additional fields like username.<\/p>\n\n\n\n
Get-Mailbox -ResultSize Unlimited | select Displayname,mailboxplan,maxsendsize,maxreceivesize | Export-Csv \"C:\\mailbox_report.csv\" -NoTypeInformation<\/code><\/pre>\n\n\n\n
We can see some (user) mailboxes are indeed set to 35MB rather than the expected 58-60MB.<\/p>\n\n\n\n
Note that the Discovery Mailbox does not have Mailbox Plan specified.<\/p>\n\n\n\n
Remediate Individual Mailbox<\/h2>\n\n\n\n
So say to set the Bob Smith mailbox to 60MB, we can just run:<\/p>\n\n\n\n
Set-Mailbox Bob Smith -MaxReceiveSize 60MB -MaxSendSize 60MB<\/code><\/pre>\n\n\n\n
And verify:<\/p>\n\n\n\n
PS U:\\> Get-mailbox \"Bob Smith\" | fl mailboxplan,maxsendsize,maxreceivesize\n\nMailboxPlan    : ExchangeOnline-bb0dc167-36ea-47e4-b38b-3705f30c6d4d\nMaxSendSize    : 60 MB (62,914,560 bytes)\nMaxReceiveSize : 60 MB (62,914,560 bytes)<\/code><\/pre>\n\n\n\n
Remediate All Mailboxes<\/h2>\n\n\n\n
To set all the mailboxes to 60MB, you would do the following, notice in this case ensuring we are not updating the “Discovery Search Mailbox” that in our case was set to 100MB.<\/p>\n\n\n\n
Get-Mailbox -Resultsize Unlimited | Where-Object {$_.DisplayName -ne \"Discovery Search Mailbox\"} | Set-Mailbox -MaxReceiveSize 60MB -MaxSendSize 60MB<\/code><\/pre>\n\n\n\n
Now verify the changes have been made as expected across all. You may want to add some additional fields like username.<\/p>\n\n\n\n
Get-Mailbox -ResultSize Unlimited | select Displayname,mailboxplan,maxsendsize,maxreceivesize | Export-Csv \"C:\\mailbox_report.csv\" -NoTypeInformation<\/code><\/pre>\n\n\n\n
Additional Information\/Further Reading<\/h1>\n\n\n\n