Phased Migration Options

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Phased Migration Options

Your plan needs to include how many mailboxes will migrate to Microsoft Exchange Server at a time. Generally, migrating the entire postoffice (every mailbox) will be easier to plan for, implement and maintain than migrating a partial postoffice. Migrating a partial postoffice is more likely to occur during a pilot or limited rollout, when the number of users is small and the issues are easier to solve or work around. The following section explains why this is so.

Migrating a Partial Postoffice

When you migrate a partial postoffice, the MS Mail (PC) 10/10/10 (Network/Postoffice/Mailbox) address is not retained, which is a disadvantage for the following reasons:

Routing to the postoffice from other postoffices and gateways must be maintained so that the remaining mailboxes continue to receive mail.
Mail sent from foreign systems through an MS Mail (PC) gateway to an old address is delivered to the old address (if the mailbox hasn't been deleted) or returned as undeliverable, so the user has to check for mail on both systems. Administrators for the foreign system need to replace the old address with the new one in the system address list, and users in that system need to update their personal address books.
Directories in MS Mail (PC) postoffices and foreign systems that have an old address have to be updated immediately because the old address is invalid. If you configure directory synchronization with Microsoft Exchange Server, this updating occurs automatically. If you want to leave the old mailboxes on the postoffice but remove them from the global address list, you can modify the mailbox. In the MS Mail (PC) Administrator program, change the Include User in Directory Synchronization option to No.
Replies from MS Mail (PC) users to mail received in the past from a user who has since migrated are delivered to the old MS Mail (PC) mailbox if the mailbox hasn't been deleted, or returned as undeliverable. Users should try to resend their mail, using the MS Mail (PC) global address list.
Mail messages that are migrated to Microsoft Exchange retain only the display name of the sender with the exception of sender and recipients that have SMTP or X.400 type addresses, which are preserved. Reply messages need to resolve this display name and match it to one in the Microsoft Exchange global address list or the PAB. If the display name has changed, or the name does not resolve to a unique entry, then the user must select the name from an address list.
MS Mail (PC) personal address book (PAB) entries of users who have already migrated are invalid. Users should try to resend their mail, using the MS Mail (PC) global address list. Users can synchronize their PAB with changes in the global address list using PABCheck.

Routing Change

No changes in the routing of e-mail from other postoffices and gateways are allowed when doing a partial postoffice migration. The mailboxes remaining on the postoffice need mail addressed to them to continue to be delivered.

MS Mail Gateway passthrough

Mail sent from external sources through Microsoft Mail (PC) gateways to the old address will be routed to the Microsoft Mail (PC) postoffice. This could cause mail to be returned as undeliverable or delivered to an unused mailbox.

To avoid this, you can update the gateway's mapping table (if there is one) so that mail is routed to the new address. For example, the Microsoft Mail (PC) X.400 gateway has an address mapping table so that mail addressed to the old X.400 address is redirected to the new MS type e-mail address of the Microsoft Exchange Server mailbox. Updating the mapping table works well during a pilot rollout, but isn't recommended when you're migrating thousands of mailboxes at once, because gateway performance will be affected.

Tip Move users who send mail to each other frequently at the same time.

PABs for users who haven't migrated

Mail sent with old PAB addresses in Microsoft Mail (PC) will try to go to the old mailbox. If the mailbox has been deleted, the message will be returned as undeliverable. The sender can try to resend it using the Microsoft Mail (PC) global address list, which may or may not have been updated yet with the new address, depending on whether the directory synchronization cycle has completed. Users can synchronize their PAB with changes in the global address list using PABCheck.

Migrating an Entire Postoffice

If you migrate every mailbox on a postoffice at the same time, you can retain the original Microsoft Mail (PC) 10/10/10 (Network/Postoffice/Mailbox) addresses as their MS type e-mail addresses. This has many advantages, as follows.

Since the addresses of the old and new mailboxes are the same, you can route mail that has an old address. You have few routing changes and no interruption in mail delivery. However, you do need to adjust the routing in MS Mail (PC) to the old postoffice so that it's indirect through the Microsoft Mail Connector postoffice. If you used a hub-and-spoke routing system, you have to make changes to only a few postoffices. The Microsoft Mail Connector has the old postoffice removed from its routing information. When mail addressed to one of the migrated mailboxes is routed to the Microsoft Mail Connector postoffice from the Microsoft Mail (PC) side, the Microsoft Mail Connector resolves the address as one belonging to a Microsoft Exchange Server mailbox and forwards the mail.
Mail that passes through a Microsoft Mail (PC) gateway to migrated mailboxes is delivered. The Microsoft Mail Connector postoffice must have an access component for the gateway. Because the addresses for migrated mailboxes have not changed, mail is routed from the gateway to the Microsoft Mail Connector and from there to the Microsoft Exchange Server mailbox.
Directories in MS Mail (PC) postoffices and foreign systems that have old addresses are valid. Therefore, mail sent from foreign systems and routed through MS Mail (PC) are delivered to the migrated mailbox without your needing to change gateway mapping tables or update directories.
Users still on Microsoft Mail (PC) can reply to mail they received in the past from a user who has migrated. MS Mail (PC) messages retain the full address of the sender. Replying to a message automatically addresses it to the same address. Mail is routed to the Microsoft Mail Connector postoffice and then on to the Microsoft Exchange Server mailbox.
MS Mail (PC) personal address book entries of users who have already migrated are valid because old and new addresses are the same.
Migrated PAB entries for MS Mail (PC) mailboxes which have been migrated are invalid. Mail is routed to the Microsoft Mail Connector, which doesn't have a postoffice configured with that address and returns the mail as undeliverable. This undeliverable mail can be readdressed from the Microsoft Exchange Server global address list and delivered normally. There is no tool that updates the user's PAB entries as changes are made in the Microsoft Exchange Server global address list. To prevent undeliverable mail, users can remove all PAB entries of MS type from their PAB after they migrate to Microsoft Exchange Server.
Mail messages that are migrated to Microsoft Exchange Server retain only the display name. Replies to these messages need to resolve the address against the Microsoft Exchange Server global address list or an accurate PAB. Mail messages that have sender or recipients with SMTP or X.400 addresses are migrated with the complete address and can be replied to without address resolution.

PABs for users who haven't migrated

PAB entries function similarly to replies. For Microsoft Mail (PC) users, their PAB addresses continue to work for migrated mailboxes because the addresses are the same.

Migrated PAB entries will work for Microsoft Mail (PC) mailboxes that have not migrated. Mail addressed with the migrated PAB to mailboxes that have since migrated will get routed to the Microsoft Mail Connector. The Microsoft Mail Connector does not have a postoffice configured with that address and will return the mail as undeliverable. This undeliverable mail can be readdressed from the Microsoft Exchange Server global address list and delivered normally.

There is no tool that updates the user's PAB entries as changes are made in the Microsoft Exchange Server global address list. To prevent undeliverable mail, users can remove all PAB entries of MS type from their PAB after they are migrated to Microsoft Exchange Server mailboxes.