5 applications for Android:

1) Aqua Mail

Aqua Mail allows you to use multiple accounts created in Gmail, Yahoo, Hotmail, FastMail, Apple, GMX, AOL, and more. It syncs all contacts on the device, saves messages to the cloud, and has many more features.

Price: Free/$3.37 per month for additional features

2) K-9 Mail

 al features under its hood include the ability to support multiple accounts; unified inbox; privacy-friendly; automatic background synchronization or push notifications and more.

Price: free

3) Newton Mail

The application allows you to view receipts, delayed sending, and much more. It also supports Gmail, Google Apps, Exchange, Outlook and is available for all operating systems.

Price: $49.99

4) ProtonMail

ProtonMail can be used in a browser, downloading the application is not required. All messages are encrypted and no one has access to them except the sender and recipient.

Price: free

5) Nine

Nine lets you sync your favorite folders, supports Wear OS, syncs calendars and contacts, advanced email search, and more. It is considered one of the best solutions for Android.

Price: $14.99

Apps for iPhone:

1) Airmail

The application supports Gmail; EWS, IMAP, and POP3 systems; filters messages by several parameters and can be used with documents from OneDrive, Google Drive, and Dropbox.

Price: $2.99 per month

2) Spike

Spike allows you to simplify business communication, making it look like regular conversations with clients and colleagues. It also allows you to leave notes and deal with more complex tasks.

Price: Free for personal use/$7 per month for corporate use

3) Triage

The app is great as the main app, works with Gmail, iCloud, and IMAP. It arranges unread messages as a stack of cards, which allows you to quickly archive unnecessary ones or quickly reply.

Price: free

4) myMail

Allows using all of the accounts at once including Hotmail, Google Gmail, Yahoo, Edison, Outlook, iCloud, Thunderbird mail, Webmail, or Mail.ru accounts. Supports all major providers and keeps all your correspondence safe.

Price: free

5) Polymail

The application unsubscribes you from unnecessary mailings, allows you to create drafts and share them. it integrates with messaging tools like Slack and Salesforce

Price: $10 per month

via smallbiztrends.com