Hi Sivaraman NR,
To satisfy your requirement, I’d like to confirm the following points:
1.Is the open source mailing system a third party email system?
2.You said “I want to move to Office 365”, do you want to migrate the mailboxes at the same time? If so, in AD environment, we suggest deploying direct sync to sync the mailboxes to Office 365. Here is an article you can refer to: technet.microsoft.com/.../hh967642.aspx.
In addition, we generally focus on three suggested methods to do the migration in AD environment with an Exchange server:
1.the cutover migration
technet.microsoft.com/.../jj159539.aspx ,
2.the staged migration
technet.microsoft.com/.../jj874018%28v=exchg.150%29.aspx ,
3.the hybrid migration
help.outlook.com/.../ff633682.aspx .
Thanks,
David Zhang