What email provider is suggested?

[gianpaj]

I set it up for testing with https://app.mailgun.com, but it doesn't have an IMAP, so Libredesk can't receive the customer emails.

Is there a free IMAP and SMTP provider that you could suggest? Even for a sub domain help.yourdomain.com

[abhinavxd]

The simplest option is Gmail with a personal account using an App Password for IMAP & SMTP.
For your subdomain you can checkout services like Zoho mail.