[Other Accounts] I only receive some emails when adding a Gmail account through the "Add External Email" feature.
Gmail offers two modes: POP Standard mode and Recent mode. Using the Standard mode for synchronization with a single email client is recommended. In contrast, the Recent mode is recommended for synchronization with multiple email clients. However, if you use the Standard mode for synchronization with multiple email clients, you may experience a situation where only some emails are received in Gmail.

In POP Standard mode, Gmail lists approximately 250 unread emails (excluding spam and trash), starting from the oldest ones. In other words, if you have connected the same Gmail account to different clients, the emails already fetched by a client will not be fetched by the next client. For more detailed information, please consult the Google Help resources.

If you want to retrieve all Gmail emails while using multiple email clients, please use the "Recent mode" in Gmail.
1. Check the "Email Address" or "Username" field in the client's POP settings page.
2. Add "recent:" before the email address. For example: recent:example@gmail.com.
3. For more information, please refer to the Google Help resources on "Email not properly downloading."