Can I use both Microsoft 365 mail and IONOS mail basic with the same domain?

[EternityIT]

Hello,

I started a small business and running a website hosted by IONOS (1and1).  Their package comes with 5 mail basic e-mail accounts and it is sufficient initially.  Now I want to subscribe Microsoft 365 Business Standard (with e-mail service) and migrate the "real" users to Microsoft 365.  On the other hand, I would like to keep the "General" email accounts within IONOS with the same domain.

e.g.   The following users (and more in the future) will use Microsoft 365 mail

         John.doe@mycompany.com
         Jane.doe@mycompany.com
         Peter.pan@mycompany.com

While the following general account will remain to use IONOS's mail basic accounts:

         info@mycompany.com
         sales@mycompany.com

Is this possible?  If it is, can you let me know how to do this or point me to the related document links?  Many thanks in advance!

Regards,
TC

[#2]

@EternityIT 

Hi TC,

I don't believe it is possible or recommended to do what you want. When I send  an email, it looks up mycompany.com MX record to know where the server to send the email to actually is - in this case either the IONOS mail servers or the Office 365 mail servers.

You can set the MX records pointing to both, but my mail server has no way of knowing if the recipent I'm emailing mailbox is on the IONOS mail server, or the 365 EOL servers, so it'll pick one and if it's the wrong guess I'll get a bounce back.

For example, I try to email john.doe@mycompany.com which is on 365 - my mail server checks your MX records, sees that it is either on IONOS or 365 and picks one. There is a 50% chance it'll be the wrong choice. Lets say it picks wrong and tries to route the email to IONOS - the IONOS email servers have no idea about john.doe@mycompany.com because it is on 365, so it pings me a bounceback saying "Sorry, no idea who john.doe@mycompany.com is, so this is the end of the road" My email server will then go "fair enough" and won't try to send the email again. The email won't get through.

The result will be very unreliable email delivery, and you'll also have issues with Autodiscover. In short, trying to do this will create far more headaches than migrating everything over to Office365 and having everything pointing to Office 365.

Hope this helps,

Mark

[#3]

@HidMov 

Hi Mark,

Thanks for the response, that's what I am afraid of.  It is pity that we have to get extra Exchange licenses for some share mailboxes.

Meanwhile, I am digging further in some MS documents like "Set up connectors to route mail between Office 365 and your own email servers".  It seems that the use of connectors can maintain mailboxes in both MS 365 and own email servers using same domain.  However, this looks like only for on-premises e-mail servers.  Will dig a little further before I give up.

Regards,

TC

[#4]

Hi @EternityIT 

Yes, it is possible to set up connections between your own on-prem Exchange server and Office 365, but my understanding of IONOS is that it is a hosted mail solution which means they are unlikely to set this up for you.

You mentioned getting extra Exchange licences for additional share mailboxes - are you referring to shared mailboxed in Microsoft 365? If so, then there is no licence cost for a shared mailbox. There are some limitations, but it is essentially a 'free' mailbox.

https://docs.microsoft.com/en-us/exchange/collaboration-exo/shared-mailboxes

https://docs.microsoft.com/en-us/microsoft-365/admin/email/about-shared-mailboxes?view=o365-worldwid...

You can also convert a normal mailbox to a shared mailbox and vice versa - this is useful for us when a client has a user who has left but they still need to keep the mailbox active/accessible without cost.

Hope this helps,

Mark

[#5]

@HidMov 

Hello Mark,

Thanks for the advice.  This sounds a prefect solution for me.  So I will have all real persons to use the mail in MS 365 and setup shared mail boxes for general email addresses (e.g. info@mycomany.com or sales@mycompany.com, etc.) that can have shared access by the MS 365 mail users.  Thanks again.

Best Regards,

TC