Email clients such as Outlook, Mail and Thunderbird allow you to export an email message. When you export an email, you get the entire message, the information in its header, the email addresses and attachments, as well as the sender's time stamps and IP addresses. Email clients have this standard functionality but web-based email and the interface they use does not have an export function. That said, they do have some sort of feature that allows you to save a message offline. With Gmail, you can save an email as an EML file and import it into most email clients. That's how.
Email Gmail come file EML
Open Gmail in your browser. Open the email you want to save offline and click the little arrow next to the Reply button. From the menu, select the 'Show original' option. This will open the message in a new window. In this new window, you will see an original Download button. This will open a Save As dialog box in your browser and the default extension it will save as is TXT. You don't want to save it with the TXT extension. Switch to EML and from the file type drop-down menu, change it from text file to all files. Save the file. It downloads in seconds. Once the file is downloaded, you can import it into any email client you want or store it on your hard drive or external drive. If your email had attachments, check if the downloaded file has attachments or not. Gmail might not include the attachment in the original message, or the attachment might come from Google Drive or maybe it was sent as a link. The point is, if you're doing this to save an attachment, this may not be a foolproof method. To import email into an email client, you must first find the import function in your client. It won't be in the same place in all email clients. Once you have the import option, select this EML file and the email will open. Alternatively, you can simply double-click the file and it will open in your default email client. If your email client supports drag & drop, you can use it to open the message in the client.