Email can be access via different means.
Nextcloud
You are able to connect your email to the embedded email client within Nextcloud. However, this is not recommended. This will cause delay, slowness, and impede on your storage quota. The server is community hosted and requires community mindset. Email is not a heavy task for the server but it can be accessed via other means. Allowing heavier tasks to be completed on the server. (i.e. file storage)
Desktop Clients
Refer to Email with orGGro for configuration settings.
Mozilla Thunderbird – Recommended
Thunderbird is the standard for email clients within the Free License Open Source Software community. Likewise, your use and donation (optional) is helpful in keeping alive an anchor in advocacy for FLOSS.
More info can be found here.
eM
If you would like to find more customization and visual functionality, eM is a client with a modern UI and clean interface. The software is proprietary and closed source. The software is free to use for two connected emails.
More info can be found here.
Microsoft Outlook
The standard, boring, bloated email client. Consider yourself a traitor and next in-line to walk the plank.
Apple/Windows Mail
Similar to Outlook, Apple Mail and Windows Mail are closed source, proprietary software. Not recommended, although it will function.
Mobile Clients
Refer to Email with orGGro for configuration settings.
K9 – Mail Client
K9 is an open source client that is being rolled into Mozilla as the Thunderbird mobile app. Therefore the app follows FLOSS philosophy. Android only. Highly recommended.
More info can be found here.
Spark iOS
Spark is a closed source email client that provides some alternatives to iOS Mail. The software is closed source, considered to be Freemium, as in it is free with caveats.
More info can be found here.
iOS Mail and Android Mail
Both are closed source, proprietary software. Not recommended, although it will function.