It is always better to have a set of testing plan to be done after we complete a major task.Office 365 being  a major preferred product in the current messaging trend most of the companies decide to have office 365 in their environment.

Most of the companies would like to have a coexistence kind of setup between existing exchange 2010/2013 servers  in on-premise and office 365.Few of them keep this kind of a hybrid setup for a while to experience the office 365 service and then they slowly get transitioned to office 365.Rest of them would like to continue with the coexistence hybrid setup of office 365 and exchange on premise together always.

Administrators and end users might not be aware if there are any functionality broken after the complete migration until we execute a post migration testing plan.

Below are the list of possible post migration testing that can be performed for a healthy hybrid O365 Exchange on premise coexistence.

1) Test Email from O365 Mailbox to On-Prem mailbox account.
2) Test Email from On-Prem mailbox account account to O365 Mailbox.
3) Test Email from O365 account to external Gmail/hotmail account.
4) Test Email from external Gmail/hotmail to O365 Mailbox.
5) Test Private Meeting from O365 Mailbox.
6) Test Private Meeting from On-Prem Mailbox to office 365 Mailbox.
7) Free Busy Information of O365 visible for On-prem Mailbox.
8) Free Busy Information of On-prem Mailbox visible for O365.
9) On-Prem Mailbox visible for O365 users in GAL.
10) O365 Mailbox Visible for On-Prem in GAL.
11) Accessing a shared calendar in On-Prem by O365 User.
12) Accessing a shared calendar in O365 by On-Prem User.
13) Test Email to O365 account from Iphone/Andriod device through activesync.
14) Test Email from O365 Account to Iphone/Andriod Device through activesync.
15) Mail Contacts of On Prem Visible for O365 and Vice Versa.
16) OOF of On Prem Account Visible for O365 and Vice Versa.
17) Mail Delay Delivery from O365 to On prem and Vice Versa.
18) Granting Delegate access for On-Prem Mailbox to O365 Mailbox and Vice Versa.
19) Send email to distribution list from O365 to On prem and Vice Versa.
20) Meeting Requests Forward from O365 to On-Prem and Vice Versa.
21) Setting up E-Fax Account creation for O365 Mailbox.
22) Application related Relay setup for O365 Mailboxes where the application resides in onpremise.
23) Email Encryption setup for O365 Mailboxes.
24) External OWA Setup(limitations if any) and testing for O365 Mailboxes.
25) Recover email from Recover Deleted Items Folder.

Also refer :

Post-deployment checklist for Office 365 –  http://technet.microsoft.com/en-us/library/hh852456.aspx

Cheers

Sathish Veerapandian

Technology Evangelist