On Office 365, by default, the maximum size of a mail is 36MB, you can increase message size limits up to 150MB.You can do this on a per mailbox basis, or for every mailboxes on your tenant.
Increase Message Size Limits – One mailbox
Get-Mailbox emmanuel.demilliere | Set-Mailbox -MaxSendSize 150MB -MaxReceiveSize 150MB
Increase Message Size Limits – New mailboxes
In order to modify the newly created mailboxes, you need to modify the MailboxPlan :
Get-MailboxPlan | Set-MailboxPlan -MaxSendSize 150MB -MaxReceiveSize 150MB
Increase Message Size Limits – All mailboxes
To modify all mailboxes, you can use the “Get-Mailbox -ResultSize Unlimited” and then pipe it to the “Set-Mailbox”, it can take a few minutes to complete, depending on the number of mailboxes :
Get-Mailbox -ResultSize Unlimited | Set-Mailbox -MaxSendSize 150MB -MaxReceiveSize 150MB
You can check your modifications with :
Get-Mailbox firstname.lastname@example.org | select Name,MaxSendSize,MaxReceiveSize
Once the maximum message size is increased, you need to be careful about your SMTP gateway if you use one. It may need the same kind of modification to allow your mail to go through.
By tweaking this parameter, the size of your mailboxes can grow quicker than expected, and the .ost file can have a negative impact on your hard drives.
You can read more details here.