- Mozilla Thunderbird is an E-mail program. - An e-mail program is also known as an e-mail client. - When using an e-mail client, a server that stores and delivers your messages is used. - This server is often hosted by your ISP but can be another Internet company. - An e-mail client needs to connect to a server to download a new e-mail, whereas e-mail stored online is always available to any Internet-connected device.