IMAP connections.If IMAP is or can be made available as a protocol to connect to the existing email system, an IMAP migration is possible.
if IMAP is not available on non-ExchangeServer systems, migration will need to be PST-based or use a third-party tool.
IMAP cross-premises coexistence.There is a lower probability option with existing IMAP-based mail systems for implementing cross-premises coexistence.This approach might be necessary with thirdparty email systems where there are more than 2,000 users.
POP3 or proprietary connections.If the third-party email system only provides POP3 or a proprietary protocol over which users connect to their mailboxes, the only migration options are:
o PSTmigration if the users have Outlook.
o Third-party POP3-based migration tool if they do not use Outlook.