El formato .eml es utilizado para poder descargar los correos electrónicos localmente y poder ver más información sobre estos correos.
If you have an email configured in an email client, when downloading it from there it is very likely that it will already download in .eml format by default. Even so, we show how to download it locally from the most common email clients.
Para descargar el correo desde SOGo, es necesario acceder a la cuenta de correo dónde se encuentra el correo a descargar desde mail.nombresudominio.com How to access and manage email accounts from SOGo
Once you access SOGo webmail, you can view a screen with the different elements that allow you to manage your messages.
You only need to select the email you want to download, and then go to the upper-right corner, to the "More" icon
Next, several available actions will appear; you must go to "Download message" and click on it.
You will now have the email downloaded locally on your device:
To download an email in .eml format from Thunderbird, simply select the message you want to download and right-click, then go to "Save as...". You can also select the message/messages you want to download and click on "More" shown in the lower-right corner of your Thunderbird.
A window will open to select the folder where you want to save this file and also the format type. You must open the dropdown and select "Mail files (*.eml)"
To download an email in .eml format from Outlook, simply select the message you want to download and right-click, then go to "Save as"
Outlook already assigns the .eml file type when saving it on the device; in that case, select the folder where you want to download it and click on "Save"
To download an email in .eml format from Mac Mail, simply select the message you want to download and go to the top of your Mac, in the "File" section, and then go to "Save as..."
A window will open where you can select the name under which the file will be downloaded and the location where it will be saved. In this section, you must modify the "Format" and select the "Plain text" option. Finally, you only need to Save it to have it in the selected location.
To download an email in .eml format from Gmail, you need to open del correo/mensaje que quiera descargar. Una vez esté dentro, debe ir al botón de 
en la parte derecha de su mensaje, y ahí debe clicar sobre "Download message". The message will be downloaded directly in .eml format to the folder designated for downloads in the browser.
Related links
Bounced or rejected emails: causes and how to review them
How to follow good practices when sending email
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