Decide on Office 365 Migration Path

Deciding on the best migration path of your users’ email to Office 365 can be difficult. Your migration performance will vary based on your network, existing messaging systems design, mailbox size, migration speed, and so on.

For migrations from an existing on-premises Exchange Server environment, you can migrate all email, calendar items, tasks and contacts from user mailboxes to Office 365. The available methods are cutover, staged, and Exchange Hybrid migrations.

For migrating third-party email to Office 365, you can configure mail flow coexistence if the third-party email provider permits then migrate the mailboxes using IMAP or cutover migration options.

Migrating from Exchange 2003 or Exchange 2007

Number of mailboxes	How quickly do you want to migrate?	Use
Fewer than 150	Over a weekend or a few days.	Cutover
Fewer than 150	Slowly, by migrating a few users at a time.	Staged
Over 150	Over a weekend or a few days.	Staged
Over 150	Slowly, by migrating a few users at a time.	Staged

Migrating from Exchange 2010 or Exchange 2013 or Exchange 2016 or Exchange 2019

Number of mailboxes	How quickly do you want to migrate?	Use
Fewer than 150	Over a weekend or a few days.	Cutover
Fewer than 150	Slowly, by migrating a few users at a time.	Exchange Hybrid
Over 150	Over a weekend or a few days.	Exchange Hybrid
Over 150	Slowly, by migrating a few users at a time.	Exchange Hybrid

Migrating from third-party email system to Office 365

Number of mailboxes	How quickly do you want to migrate?	Use
Fewer than 150	Over a weekend or a few days.	Cutover
more than 150	Slowly, by migrating a few users at a time.	IMAP with mail flow coexistence

If the mailboxes you’re migrating contain a large amount of data, you can also use Office 365 Import Service to import PST files to Office 365.

Office 365 MailFlow Scenarios and Best Practices

Microsoft Office 365 gives you the flexibility to configure mail flow based on your requirements and uses scenario to delivered email to your organisation’s mailboxes. The simplest way to configure mail flow is to allow Microsoft EOP to handle spam filter and Maiflow of your organisation. However, you may have already invested your infrastructure handle mail flow. Microsoft also accepts this situation and allow you to use your own spam filter.

The below scenario and use cases will allow you to determine how you can configure MailFlow of your organisation.

Mailbox Location	MailFlow Entry Point	Scenario & Usecases	Recommended MailFlow Configuration  and Example MX record
Office 365	Office 365	Use Microsoft EOP Demote or migrate all mailboxes to office 365 Use Office 365 mailboxes	MX record Pointed to Office 365 MX: domain-com.mail.protection.outlook.com SPF:  v=spf1 include:spf.protection.outlook.com -all
On-premises	On-prem	Prepare the on-prem to be cloud ready Build and Sync AAD Connect Built ADFS Farm	MX record Pointed to On-prem MX1.domain.com SPF: v=spf1 include: MX1.domain.com  include:spf.protection.outlook.com -all
Third-party cloud, for example, G-Suite	Both third-party and office 365	Prepare to migrate to Office 365 Stage mailbox data MailFlow co-existance	MX record pointed to third-party cloud MX record Pointed to On-prem in.hes.trendmicro.com SPF: v=spf1 include:spf.protection.outlook.com include: in.hes.trendmicro.com include: ASPMX.L.GOOGLE.COM -all
Combination of On-premises and Office 365	On-premises	Hybrid Environment Stage mailbox migration MailFlow co-existance	MX record Pointed to On-prem spam filter MX record Pointed to On-prem MX1.domain.com SPF: v=spf1 include: MX1.domain.com  include:spf.protection.outlook.com -all
Combination of On-premises and Office 365	Third-party cloud spam filter	Hybrid Environment Stage mailbox migration MailFlow co-existance	MX record Pointed to third-party cloud spam filter MX record pointed to third-party cloud MX record Pointed to On-prem in.hes.trendmicro.com SPF: v=spf1 include:spf.protection.outlook.com include: in.hes.trendmicro.com -all

MailFlow Configuration Prerequisites:

1. Make sure that your email server (also called “on-premises mail server”) is set up and capable of sending and receiving mail to and from the Internet.
2. Check that your on-premises email server has Transport Layer Security (TLS) enabled, with a valid public certification authority-signed (CA-signed) certificate.
3. Make a note of the name or IP address of your external-facing email server. If you’re using Exchange, this will be the Fully Qualified Domain Name (FQDN) of your Edge Transport server or CAS that will receive an email from Office 365.
4. Open port 25 on your firewall so that Office 365 can connect to your email servers.
5. Make sure your firewall accepts connections from all Office 365 IP addresses. See Exchange Online Protection IP addresses for the published IP address range.
6. Make a note of an email address for each domain in your organisation. You’ll need this later to test that your connector is working correctly.
7. Make sure you add all datacenter IP addresses of Office 365 into your receive connector of on-premises Exchange server

Configure mail to flow from Office 365 to your email server and vice-versa. There are three steps for this:

1. Configure your Office 365 environment.
2. Set up a connector from Office 365 to your email server.
3. Change your MX record to redirect your mail flow from the Internet to Office 365.

Note: For Exchange Hybrid Configuration wizard, connectors that deliver mail between Office 365 and Exchange Server will be set up already and listed here. You don’t need to set them up again, but you can edit them here if you need to.

1. To create a connectorExchange in Office 365, click Admin, and then click to go to the Exchange admin center. Next, click mail flow click mail flow, and click connectors.
2. To start the wizard, click the plus symbol +. On the first screen, choose the appropriate options when creating MailFlow from Office 365 to On-premises Server
3. Click Next, and follow the instructions in the wizard.
4. Repeat the step to create MailFlow between On-premises to Office 365.
5. To redirect email flow to Office 365, change the MX (mail exchange) record for your domain to Microsoft EOP, i.e. domain-com.mail.protection.outlook.com

Relevant Articles:

Mailflow Co-existence between G-Suite and Office 365 during IMAP Migration

Office 365 Hybrid Deployment with Exchange 2016 Step by Step

Centralized MailFlow: NDR Remote Server returned ‘550 5.7.1 Unable to relay’