Reality is that most of us have more than Two email Accounts :(
We really need a tool to sync all Contacts and Calendars frum multiple email accounts - No Need to sync email.
I do NOT see a need to sync email, since forwarding to and managing all emails from one account works very well. True, you may want to provide a tutorial on how to do that. But, the REALITY is that the Existing Methods Work very well.
Seem it would be best to Provide a Feature for the User to Select the Primary Account. then, everything would sync with it.
Since multiple passes could be required, I suggest letting the User select the Sequence and number of iterations. While I have multiple accounts, I use one most of the time. So, one pass through others would be sufficient for me :)
Keep it Simple :)
Gmail, iCLoud, and Outlook seem common sources. If it is possible to provide a Generic Method to add other email providers, like Yandex, mail.ru ... that would be practical and increase the audience significangly.
I would like this option, it seems that it might be possible to sync contacts from more than one google account. I have several email addresses and each one has a separate folder which maps onto a separate .ost file in Outlook, so I guess you could synchronise per folder?
I strongly support the possibility to sync to more than one Google Account - nowadays there is often a need to differentiate between work and private phones and hence Google accounts.
Moreover, a filtering based on Tags ("categories" in Outlook) would be useful - so you could e.g. sync just work category contacts with a specific Google account.