Changing the email send address in Office 365 without moving your domain

Using Office 365 Exchange Online for your email while leaving the hosting with your existing hosting provider is not as straightforward as one might expect.

Reasons for using Office 365 while staying with my current email hosting provider are:

• I want to use Office 365 Online because of its higher storage limit, to manage tasks integrated with my Windows Phone (see: Synchronize Windows Phone with Outlook Emails, Contacts, Calendar and Tasks) – the Mango beta is supposed to include a Tasks app – and to move away from Gmail (because I do not like them scanning my e-mail).
• I want to stay with my current hosting provider internet24.debecause they have worked flawlessly for years and I want my emails in redundantly in at least two locations as a precaution for service outages. Remember: the 99.9% availability guaranteed by Office 365 allows for ~8h/year downtime.
• Simply connecting from Office 365 to my existing email server via IMAP is not an option, because I do frequently exceed the storage limit of my current e-mail hosting provider which causes the server to delete old emails – which sadly results in those messages being deleted on the client too(!).

Migrating from Gmail to Office 365 while staying with current email hosting provider

With Office 365 you can not simply configure a reply-to address (like you can in Gmail). My e-mail domain is petermeinl.de. My Office 365 domain is leanwork.onmicrosoft.com.
By default Office 365 uses pit@leanwork.onmicrosoft.com as send address for my emails and recipients will reply to this address. I wanted to change the send address to pit@petermeinl.de.

I used the following steps to change my email send address from pit@leanwork.onmicrosoft.com  to pit@petermeinl.de:

1. Verify email domain petermeinl.de in Office 365 using “Admin, Domains, Add a domain“. Cancel the wizard after verifying the domain and before moving it (don’t click Next)!
2. Install Microsoft Online Services Module for Windows PowerShell.
   For background information and prerequisites see Use Windows PowerShell to manage Office 365
3. Change the email send address via PowerShell

import-module MSOnline
$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://pod51013psh.outlook.com/PowerShell-LiveID?PSVersion=2.0 -Credential $LiveCred -Authentication Basic -AllowRedirection
Import-PSSession $Session
Set-Mailbox  pit@leanwork.onmicrosoft.com -EmailAddresses SMTP:pit@petermeinl.de, pit@leanwork.onmicrosoft.com

To copy my existing emails to Office 365 and to keep get new ones I:

1. Copied all existing emails from Gmail to Office 365 via the Exchange email migration tool for IMAP mailboxes.
2. Configured email forwarding from internet24.de to Office 365 via the internet24.de admin site.