The best alternative to BlueMail is Betterbird, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to BlueMail and eight of them are so hopefully you can find a suitable replacement. Other interesting alternatives to BlueMail are FairEmail, Talanoa, Aqua Mail and myMail.
Betterbird, a soft fork of Mozilla Thunderbird, improves user experience with advanced features, bug fixes, security patches, and multi-row descriptions similar to Outlook. Compatible with multiple platforms and can be used alongside Thunderbird, sharing the same profile.
Open-source, privacy-focused email client offering features such as encryption via OpenPGP and S/MIME, unlimited accounts, a unified inbox, customizable settings, and reformatted emails to prevent phishing. Supports two-way synchronization and provides Pro features for additional customization.
Talanoa is the first inbox designed around people. Cut the noise, focus only on what matters.
AquaMail is an email app for Internet and Exchange mail, for Android 4.0.3 and higher.
No-frills, easy to use, easy to maintain Email client for Android based on the JSON Meta Application Protocol (JMAP).
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BlueMail is more good-looking. But Betterbird is much more powerful with folders (per account or cross-accounts in local), filters, add-ons (the sames as Thunderbird) and so on. I'd rather use Bluemail for one or two mail accounts and no need of powerful features, and Betterbird for many email accounts and powerful features. For instance, you can put in one single folder all the mailing list you have subscribed, even with different accounts.