You can create a shared mailbox in Office 365, for addresses shared among users, such as for Sales, Info, or Billing. Assign Send As permissions to users and they can send mail or reply to messages using that address. The mailbox will be added to their profile automatically. Note that these mailboxes do not require a license, so there is no extra charge for them.
You can create shared mailboxes in the Office 365 console or using PowerShell. If you want to set a lower mailbox quota, you'll need to use PowerShell.
As of October 2014, the limits on shared mailboxes in the Small Business and Enterprise plans was raised to 50 GB. Shared mailboxes in Exchange Online Plan 1 and Plan 2 have a quota of 10 GB.
Enter the following commands in PowerShell to log into Office 365
$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection
Use this code then rerun the Import-PSSession $Session from above. If you did not receive the error, you are ready to create the shared mailbox.
Create the Shared Mailbox:
New-Mailbox -Name "Account Manager" -Alias accounts -Shared
To set a lower quota:
Set-Mailbox accounts -ProhibitSendReceiveQuota 10GB -ProhibitSendQuota 9.5GB -IssueWarningQuota 9GB
Assign FullAccess right to either a user or a security group. If you are giving full access to a group of people, use a security group because it is much easier to manage. Use the user alias only when a very limited number of people will need rights. When a user has full rights directly, the mailbox will show up in the profile automatically as a second mailbox; group members will need to add the mailbox to their profile in Account Settings, double click on account, More Settings.
Add-MailboxPermission "Account Manager" -User AcctMrgDG -AccessRights FullAccess
Give the same user or group Send as rights:
Add-RecipientPermission "Account Manager" -Trustee AcctMrgDG -AccessRights SendAs
When you are finished, you should end the session before closing the powershell window, otherwise your credentials are "live" for 15 minutes after the window is closed.
To check the configuration, use these cmdlets:
Get-Mailbox alias Get-RecipientPermission alias
GUI Tool from Office 365
The Office 365 product team released a GUI-based tool to make it easier to create and configure shared mailboxes. You need to create the create a security group then run the tool to create a shared mailbox and assign FullAccess and SendAs permissions to your security group. Download the tool from Create Shared Mailboxes with GUI-based Tool. Note that this tool only works with security groups, not user accounts. If you want to create a shared mailbox and assign it to a user, you need to use the PowerShell commands above.