I would like to recommend Iredmail and Zhang Huangbin. We had a difficult migration to perform due to the age and lack of resources of our old mail server.  We engaged Zhang to do the migration for us.  Zhang was very responsive to any issues that arose and closely monitored the transfer of data which occurred over a period of 24 hours or more.

We received prompt remote support over the following days and he quickly resolved a number of unexpected issues that arose.  Zhang communicated regularly during the first two weeks after installation to make sure we were happy with the new mail system, and was pro-active in identifying and fixing problems.

Zhang has excellent written English so there was no difficulty with communication during the whole process.  I would highly recommend using Zhang's services to carry out a migration to Iredmail.