Hello, there are several ways to retrieve the list of recipients. For example:
a) Retrieve message info (ImapMessageInfo object) using GetMessageInfo method. Use ImapMessageInfo properties such as To or CC to get a list of recipients.
b) Download the message into MailMessage object using GetMailMessage method. Use MailMessage properties such as To or CC to get a list of recipients.
In both cases, the To and CC properties return an instance of MailAddressCollection that can easily be iterated.
The properties of MailMessage can be modified, but please be aware that this won't change the message at the IMAP server. 
Unfortunately, the IMAP protocol doesn't make it possible to modify a message at the server, so if you would like to achieve something like that, you have to download the message, modify it, upload it again (it will get a different ID) and delete the original one.