- For Ubuntu and CentOs -

1) Ensure that the new mailbox is provisioned with enough space to hold all of the old email. By default emails in Sentora are only allotted 200MB. To allow more space for all email account navigate through menu as such  Admin -> Module Admin -> Mail Config and change the "Max Mailbox Size" to your liking.

2) Install imapsync :

• Centos:  yum install -y imapsync
• Ubuntu: apt-get install -y imapsync

3) use this command
imapsync --host1 mail.domain1.tld --user1 john@domain.tld --password1 thepassword --host2 mail.domain2.tld --user2 john@domain.tld --password2 thepassword
where :

• --host1 argument specifies the old mail server FQDN
• --user1 argument specifies the old email found on the old email server
• --password1 argument specifies the password for the old email account
   
• --host2 argument specifies the new mail server FQDN
• --user2 argument specifies the new email found on the new email server
• --password2 argument specifies the password for the new email account

(From aareya response here, thank you !)