Split Domain and Sync, Mirroring or Multi Server

[hadirs]

Here The Setup

Domain = example.com
host A = mail.example.com
Server = Microsoft Exchange (1000 Users)
IP = 1.2.3.4

user 1 = foo@example.com
user 2 = bar@example.com

host B = webmail.example.com
Server = Zimbra Network + Zimbra Mobile (400 Users)
IP = 1.2.3.5
user 1 = foo@example.com

external links
Split_Domain
split-domain-os-x-postfix
Migration issue with split domain & multi-servers

all users only know mail.example.com but want to get mobile service and webmail from zimbra,

my issue is
how to make the user foo@example.com can open his old mail box with ton of mail and spam at both mail server (in this scenario webmail.example.com and mail.example.com) (*** i don't like migrate his mail to new mail server because his 3 gig mailbox with rule of folder filter and spam is to big effort for the looser guy like him (4year archive***)

ext;
like forward mail for "some users" @example.com in host mail.example.com to "existing users" @example.com in host webmail.example.com but keep the mail copy so the users can have identical mailbox in mail.(ME) and webmail.(Zimbra)

in the external link,

user foo@example.com at mail.example.com didn't got any mail because all mail being re-route to foo@example.com at webmail.example.com

previously i used imapsync to mirroring, but this make un-sync because not real time sync