How does Mithi SkyConnect co-exist with Microsoft Office 365 or Hosted Exchange on the same SMTP Address space (email domain)?

What is the need? :

There is a requirement to host the email infrastructure of a company across 2 different solutions, such that both the solutions share the same SMTP address space and the company can divide their mailboxes across the two solutions. This is typically done to allow a set of users to use one of the solutions for the value that it offers and allow the remaining users to derive benefit from the value delivered by the other mailing solution.

Essentially your critical requirements are not met entirely by one solution, but the benefits offered by that solution cannot be ignored either. In the case of Microsoft Office 365/Microsoft Hosted Exchange, some of the key management people, may need to use the Microsoft solution features and may need tighter integration with their Microsoft profiles etc, which will not be available with the Mithi solution. At the same time, the remaining set of users, have no need for all the capabilities of the Microsoft solution and it would be most effective to host them on a simpler solution.

A Co-existence solution, as the name suggests, is a solution where 2 or more mailing systems work seamlessly together to provide a common/ shared address space (email domain name like, and if possible a centralized directory for authentication and personal properties of entities (users, groups), and common contacts of all mailing systems to all users.

Such a solution allows the organization to host separate groups of users on different systems depending on their usage requirements. E.g. in an organization with 1000 users, a typical co-existence between Mithi SkyConnect and Microsoft Office 365 will allow the organization to host 100 users on O365 and 900 on Mithi SkyConnect, sharing a common domain name in their email ids, and with seamless mail flow between the users of both systems.


Why co-exist

  • It provides the features/ functionalities required by the critical people in a way they would want.
  • It also enables the organization to optimize on the overall infrastructure, management and upgrade costs, by ensuring that a majority of the users remain on a flexible, reliable and secure cloud platform, which reduces TCO.
  • It brings in value from the open world, which gives the business an IT edge, like mobility for almost all types of devices, access to free clients and applications like Thunderbird, Jabber chat clients etc. E.g. With open technologies, you could use any Android based phone/ pad/ tab, iPhone, iPad, to get seamless access to email, calendar, chat, and address book, while on the move. A possibility like this allows the field force to be more connected to each other and not just the top management.

How does Co-existence work?

To explain the working of co-existence, we have assumed an example domain of and also assumed that there would be 100 users on Microsoft Office 365 and 900 users on the Mithi SkyConnect platform. The architecture is based on a loose coupling design, where the routing between the two systems happens via the Internet on the respective Mail Exchanger (MX) systems.

For this setup, we need to add a sub domain on O365 like

For the sake of understanding the flow, let us assume two users viz. o365user, whose mailbox is hosted on Microsoft O365 and mithiuser, whose mailbox is hosted on Mithi SkyConnect.

Solution Domain hosted Sample User
Microsoft Office 365 o365user
Mithi SkyConnect Mithiuser

Basic Setup :

  • On MS O365, provision the domain and configure the 100 users, who will use the Microsoft o365 setup.
  • On MS O365, ensure that each of these users are configured to have a “reply to” id with the domain (E.g. [email protected] will have a reply to id of [email protected])
  • On Mithi SkyConnect, provision the domain and configure the remaining 900 user accounts, which will access their mailboxes from the SkyConnect setup.
  • On Mithi SkyConnect, add and configure the remaining 100 accounts (whose mailboxes are on MS O365) to forward mail to the corresponding email id on the sub domain viz. (e.g. mail meant for [email protected] will be forwarded to [email protected])

Inbound Mail flow :

Inbound mail flow constitutes mail received from all external domains.

The MX for the domain will land on Mithi SkyConnect. This means that inbound mail for all 1000 users will land on Mithi SkyConnect. Mail destined for any of 900 users, whose mailbox is on Mithi SkyConnect will be delivered and stored locally, whereas mail destined for any of the 100 users on O365 will be forwarded to the corresponding email id on the sub domain (e.g. inbound mail for [email protected] will be forwarded to [email protected])

The mail destined for the 0365 users will be routed as outbound traffic via the Internet.

Local Mail flow on domain :

  • O365 to O365: A user of the MS O365 system sends a mail to another user on the O365 system. This mail is routed internally within O365 and delivered to the recipient.
  • O365 to Mithi: An acmecorp user of the O365 system sends a mail to another acmecorp user on the Mithi system (e.g. [email protected] sends a mail to [email protected]). Since the recipient domain,, is unknown to O365, the O365 system will treat this as an external recipient and will route this mail as outbound via the Internet to the MX receiver of, which is Mithi SkyConnect, and which accepts the mail and delivers it to the recipient’s mailbox.
  • Mithi to O365: An acmecorp user of the Mithi system sends a mail to another acmecorp user on the O365 system. The Mithi system will attempt to deliver the mail to the recipient on the Mithi system, who is configured to forward the mail to the corresponding email id on the sub domain. E.g. if [email protected] sends a mail to [email protected], the mail delivery configuration of this user instructs the Mithi system to forward the email to [email protected]. This is routed to the Internet as an outbound mail and is received via the O365 system on the MX landing points and is delivered to the mailbox of the [email protected] mailbox user locally
  • Mithi to Mithi: An acmecorp user of the Mithi system sends a mail to another acmecorp user on the Mithi system. This mail is routed internally within the Mithi server and delivered to the recipient.

Outbound Mail flow :

  • O365 to Internet: An acmecorp user on O365 sends a mail to an external recipient. This mail is routed to the Internet directly from the O365 system and carries a reply to configuration of the [email protected]. This is so that all replies are received on the Mithi system.
  • Mithi to Internet: An acmecorp user on Mithi sends a mail to an external recipient. This mail is routed to the Internet directly from the Mithi system.

Virus and Spam control :

  • Since the MX of is landing on Mithi SkyConnect, the spam and virus scanning for all the mail is done by the Mithi SkyConnect system.
  • Outbound mail from MS O365 may be scanned for Spam/Virus by the O365 system.
  • Outbound mail from the Mithi system are not scanned for spam.
  • The virus control engine is enabled on the Mithi system and all mail (inbound, outbound and local) are scanned for viruses.

Deliver-ability :

To ensure that the recipients can perceive the hybrid mailing system to be one, it is important to configure the SPF record of to contain the IP addresses of the outbound relay servers from both MS Office365 and the Mithi SkyConnect setup. The SPF record has to be configured with your DNS service provider.

Address Books :

The Mithi users will see the entire address book since all the 1000 users are added to the directory (although 100 of them will not have their mailboxes there). This makes it possible for the users of the Mithi system to see a complete global address book comprising of all 1000 users. As for the Office365 users, they will only see an address space of 100 users unless you can populate the Global address list on Microsoft Office 365 with all the 1000 users and also maintain this during provisioning (Adding and deleting users)

Authentication :

The users on O365 and Mithi will maintain their own passwords in the respective directories and via the respective interfaces. The password policies etc will apply from the separate systems respectively.

Email Archival :

Mithi is the creator of Vaultastic – the easiest and safest cloud email archiving service, which has the capability to archive mail from both Microsoft Office 365 and Mithi SkyConnect (in addition to Google Apps/G Source, Hosted Microsoft Exchange, and others). To enable archival for the 1000 users of, we would need to configure Vaultastic to receive archival mail from the 100 users on Microsoft 0365 and separately from the 900 users on Mithi SkyConnect.

Configuration Summary:

Solution Domain MX records Users
Microsoft Office 365 Point the MX records of “” to the Microsoft Office 365 inbound host names as specified by Microsoft Provision 100 users, having email id as [email protected], each having a “reply to” id configured as [email protected]
Mithi SkyConnect Point the MX records of “” these to the Mithi SkyConnect inbound host names as specified by Mithi Provision 1000 users, having email id as [email protected]. Set forwarding of the 100 users whose mailboxes are on O365, to forward mail to [email protected]


In this architecture, the end user will have to create or remove users separately from Mithi and Office365. Is there a possibility to have a single address book.
In this design, which is a loose coupling design, there is no way to have a single address book. Just to clarify,
– when a new user joins on 0365, the user will have to be added on Mithi SkyConnect and 0365.
– when a new user joins on Mithi SkyConnect, the user will have to be added on SlyConnect and in the GAB of o365.

Groups: What if there is mix of users from Office365 and Mithi to be added to same distribution group or list?
This is not a problem on either system. Once the mail flow between the users of both systems is working fine, the distribution of mail via the DLs will also work fine.

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