Post Office Protocol or POP for email accounts
An explanation on Post Office Protocol
POP, in computer science stands for Post Office Protocol. It is the way email accounts can be remotely controlled to retrieve or send email. Email programs or clients like Mozilla Thunderbird and Microsoft Outlook use the POP email service to be able to function as a stand-alone email management program. However, the POP service needs to be enabled by your email service provider for these programs to access your email account via POP service.
When this service is activated, you can access your mail – that is you can receive and send emails, without opening an internet browser. You only need to open your email program (thunderbird, outlook, etc.) and then all your emails come in those programs. You can even access many email accounts at the same time in the same program.
However, this is also the process email spammers and hackers use your email to send unsolicited emails or spam. To access an email account via POP, you need to provide the email address and the password to the program. So when you suspect someone accesses your email address through this method, change your password. If you do not want to use POP to access your email accounts anyway, disable it from your account settings so that nobody can have remote access to your account.
Yahoo mail offers this service, but only in yahoo plus. You have to pay for it so that you can use their POP service. They used to provide that service for free. Gmail on the other hand has faster user interface, better spam protection, and provides the POP service for free. It also allows you to access email accounts via IMAP.
As for the email clients that you use, it is best to get them from a reputable source. Programs like these can be reprogrammed to gather personal information. Hackers can use these email clients to get your username and password, they can steal your identity and even your cash. This is especially true for open source software and hacked software you get from illegitimate sites like torrent sites. Be warned too that I used to use a portable version of Mozilla Thunderbird and during that time, I had instances when my account mysteriously sends spam email to people in my contact list.