複数のGmailを切り替える: what it does and where it breaks down

Most people arrive at this question with a specific incident behind them. A reply went out from the wrong Gmail address. A calendar invite opened under the personal account and the work calendar was nowhere in sight. A link from Slack landed in an inbox that was not the one the message was about. The account switcher in the top right corner had been doing its job for months, and then it quietly did not. Understanding why requires knowing what that switcher is and, more importantly, what it is not.

What the account switcher actually is

Google's multiple sign-in is not a separation feature. It is a multiplexer built on top of a single browser storage area. Google states the purpose plainly:

If you have more than one Google Account, you can sign in to multiple accounts at once. That way, you can switch between accounts without signing out and back in again. Source: support.google.com

The mechanism shows up in the address bar. Open a second account and the Gmail URL picks up a session index: the first account sits at /u/0/, the second at /u/1/, the third at /u/2/. Those numbers are positions in a list held by one set of cookies, not separate identities. Every signed-in account shares the same cookie jar, the same extension set, the same download folder, and the same browser history.

That design has a consequence worth stating early. Nothing about multiple sign-in prevents one account from knowing about another. It was never built to. It was built so a person holding three Google accounts would not have to type three passwords a day. Judged against that goal it works well. Judged as a boundary between a client's data and a personal inbox, it was never the right tool, and no amount of careful clicking turns it into one.

The index is positional, not permanent

The session index reflects sign-in order, not identity. Sign out of everything and sign back in with the accounts reversed, and the account that used to be /u/1/ becomes /u/0/. Bookmarks that point at /u/1/ then open the wrong mailbox. This is the first thing that surprises people who have carefully bookmarked their way around the problem.

The parts that genuinely work

Inside a single tab, once the correct account is loaded, the switcher is solid. Labels, filters, signatures, keyboard shortcuts, and search all operate on the account shown in the top right. Drive, Calendar, and Docs follow the same index, so /u/1/ in Gmail and /u/1/ in Calendar are the same person. Composing, replying, and forwarding stay inside that account. Filters written for one account never fire in another.

The switcher also handles the case of two accounts under the same organisation cleanly. A Google Workspace account and a personal @gmail.com account can be signed in at once without either being logged out. So can two Workspace accounts from two different companies. There is no documented ceiling that most people hit in ordinary use.

What holds all of this together is the browser tab. As long as the interaction begins from a tab that is already showing the right account, the behaviour is predictable. Every failure described below happens when something originates from outside that tab.

Consolidation inside Gmail has its own ceiling

A related feature is sometimes mistaken for switching: pulling other mailboxes into one Gmail account so there is nothing to switch between. Google caps that at five, stating that up to 5 email addresses can be added to a Gmail account. For someone running two Gmail accounts and one domain mailbox that ceiling never comes into view. For someone holding a mailbox per client it arrives quickly, and the accounts that do not fit go back into the switcher, which puts the reader back where they started with a more complicated setup.

Break one: the default account decides what it cannot know

Google assigns one of the signed-in accounts a special role:

Your default account is the one you signed in with first. Source: support.google.com

The default account is the fallback for every situation where Google cannot tell which account a request belongs to. Opening a new browser window is one such situation. Google's own help page describes the ambiguity directly, noting that when two accounts are signed in and a new browser window opens, the correct account cannot be determined, so settings from the default account may be applied.

In practice this shows up as a link problem. A Google Docs URL pasted into Slack, a Calendar invite arriving by email, a shared Drive folder sent by a client: each of these is a bare URL with no session index. The browser opens it at /u/0/. If the document was shared with the account sitting at /u/2/, the result is a permission denied screen, and the fix is manually editing the number in the address bar or clicking through the account chooser. Several times a day, the tax adds up.

Break two: everything that starts outside the browser

The switcher lives inside a browser tab, which means it has no authority over anything that begins elsewhere. A notification from the macOS notification centre, a link clicked in Mail or Messages, a shortcut in the Dock, a search result opened from Spotlight: all of these hand a URL to the default browser with no account context attached.

Google documents the same class of problem in a different product. On Google Play, the account in use determines the store country, and the help page states that when accounts from different countries are signed in at once, content from the default account's country may appear. The specific service changes but the pattern does not. When ambiguity exists, the default account wins.

The phone is a separate system, not an extension of the desktop

The Gmail mobile app handles several accounts at once through its own mechanism, with a swipe or a tap on the avatar moving between them. It does not share the desktop's session index, and it does not inherit the desktop's default account. That independence is useful, and it also means any rule established on the Mac has to be established again on the phone. A person who has carefully arranged which account is default in the browser will still find the phone opening whichever account it opened last.

This is also why the switcher gives no help with attention. Every account shares one window, one badge count, and one favicon. There is no visual difference between a tab holding a client inbox and a tab holding a personal one, so the sorting happens in the reader's head rather than on screen. Approaches that give each account its own window exist precisely to move that sorting out of the head, and pages like Workspaces describe the shape that takes.

Break three: the third-party mailbox route closes in January 2027

A common workaround has been to stop switching and consolidate: pull a Yahoo or Outlook mailbox into one Gmail account, then reply from that address. Google has announced the end of that path. The help page is specific:

The "Send as" feature will no longer be available. This feature allows you to send email from a third-party address, such as @hotmail.com, @yahoo.com, and others. Source: support.google.com

The same change removes Gmailify and POP fetching from third-party accounts, with Google stating that Gmail will no longer fetch emails from third-party accounts into a Gmail account with POP. Three things are explicitly not affected: access through the Gmail mobile app for Android, iPhone, and iPad, third-party email clients using IMAP or POP, and sending from other Gmail addresses or Google Workspace aliases.

The practical reading is narrow. Consolidation still works if every address involved is a Gmail or Workspace address. If the set includes an Outlook, Yahoo, or personal-domain mailbox handled through Gmail's web interface, that arrangement has an expiry date, and any setup built now should assume it.

Break four: delegation is a different tool, not a stronger switch

Gmail delegation gets suggested as an answer to account juggling, and it solves a different problem. The limits are documented: a personal @gmail.com account can add up to 10 delegates, and a work or school account can add up to 1,000. What a delegate can do is defined narrowly:

A delegate is someone who can read, send, and delete emails in your account for you. Source: support.google.com

Two details matter for anyone considering it as a switching replacement. When a delegate sends a message from the account, their email address appears, so the recipient sees who actually typed it. And a delegate cannot chat or change the password, with a further list of features unavailable to delegates including Meet, Smart Compose, and Google Account settings.

Delegation is built for a shared mailbox with an owner and an assistant. It is not a way to run a second identity of your own with fewer clicks, and treating it as one produces surprised recipients.

The boundaries available, and what each one costs

The options differ mainly in where the line sits and what has to move for a switch to happen.

Approach	Separation unit	Outside links land at	Origin of a notification
Google account switcher	Session index in one cookie jar	Default account	Not distinguishable
Chrome or Safari profile	Browser window	Whichever profile window is frontmost	Window it belongs to
Two separate browser applications	Application	Default browser only	Application icon
A browser that keeps each web app in its own window	Window per app and account	Window bound to that account	Window and icon

Chrome's documentation describes the profile boundary in one line, stating that profiles keep Chrome information separate, including bookmarks, history, passwords, and other settings. That is a real storage boundary rather than a list position, which is why a profile survives the link problem that defeats the switcher. The cost is that profiles are managed as windows, so a second account means a second window, and link routing still follows whichever window the operating system considers frontmost.

Tools that assign a window per app take that further by binding the account to the window rather than to a browsing session, which is the axis on which products in this category differ from each other. The trade-off is memory use and the work of an initial setup. Comparisons such as Compared with Wavebox exist because the pricing and the feature lines vary more than the underlying idea does.

What to change first

Start by counting how many times a day a link opens under the wrong account, because that number decides whether this is worth any effort at all. If it is more than a handful, move the accounts that carry real consequences out of the switcher and into separate storage areas first, and leave the low-stakes accounts where they are. SpaceDeck is one option in that category for macOS, alongside browser profiles, which cost nothing to try.