Roundcube is the webmail client (IMAP client) component of selfhosting an email server. You could use it together with postfix (SMTP Server) and dovecot (IMAP and POP3 server) to put together a simple mailstack. Use opendkim and opendmarc to improve your deliverability and make it harder to impersonate your mail domain. Add spamassassin to get rid of some spam.
Another option is to look into mailcow or mailinabox, they deliver a completed package built around the above.
Do everything right and still see google and microsoft send you straight to spam because you don't have enough email traffic to be considered legitimate (or important) enough to care about.