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I have my personal accounts that I have setup on Thunderbird. I have also setup my work account which uses GMail via OAuth2. My question is, do these accounts share data of any kind between them or are they isolated? Should I use for example another Thunderbird profile or E-mail client for my work account and another for my personal accounts to maintain the privacy between these accounts?

Do the email provider (GMail, Outlook, custom, etc.) or login protocols (OAuth2, SSL, etc.) play a role in this?

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The accounts are all accessed and managed through the same Thunderbird process, so in that sense some sharing is unavoidable; if there's a bug or a security exploit[1] via your personal account, this could in theory result in exposing data from your work account. The use of things like OAuth mitigates the risk slightly - no passwords need to be stored - but an attacker might still get access to both all local data (locally stored emails both sent and received, contacts, event invites, list of addresses you use, etc.) and to the access tokens necessary to interact with the servers (sending, searching, or fetching mail, possibly also accessing other functionality depending on the scope of the access token). SSL/TLS doesn't matter at all except against network attackers, who are by definition outside of the account. Each account does connect to the server independently (even if you have e.g. multiple Gmail accounts, they'll have distinct connections) so if one of those was using plain text, an attacker on the network would be able to intercept that account's traffic but not the traffic of any account with a secure connection.

It is possible that some minor metadata might be shared between accounts, such as your name. It also used to be common for multiple accounts to use a common SMTP server for outbound mail, but that is no longer typical; each account now specifies its own outbound server and establishes its own connection.

Basically, if you trust the security of Thunderbird the application, and of the device it's running on (which is usually the sticking point where work accounts are concerned, since personal devices are usually used for things that are higher-risk than work machines and also have less monitoring), you can expect that there wouldn't be any meaningful leakage of data between accounts in Thunderbird. There's nothing enforcing this boundary beyond that Thunderbird has no reason to share data between accounts, though; a flaw in the program could result in leakage.

If you want to use Thunderbird for all email, but maintain isolation, that's what Profiles are for. See https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-thunderbird-profiles, which talks more about this feature, but the short version is that you can have one common installation of Thunderbird but multiple instances, each of which have fully separate data storage (and are running as separate processes). This means you can't view multiple mailboxes in one view, and mail sent from one account needs to be composed in the relevant process, but it does create a greater degree of isolation; all data for the accounts is stored in different locations on your file system, and each profile only has access the relevant subset of account data.

[1] Of course, if Thunderbird or your system in general isn't secure - if it's possible for an attacker to get code execution within your user context - then nearly all security bets are off; the attacker could easily access all your data for all accounts, even across profiles. If your device user account isn't privileged (admin/root) you could in theory use a different local user to access the other email account, and this would protect e.g. your work account from an unprivileged exploit on your personal account, but that's a lot of inconvenience that probably doesn't even increase effective security that much.

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