Search with comma-separated value mysql

i have 2 tables customers and imap_emails. customer table contain emails of customers.

I am using PHP-IMAP to fetch email from email server and then saving to database table imap_emails.

the imap_emails table has 2 fields from and to and to field contains comma separated values.

i need to fetch emails from first table and then search against to and from on imap_emails.

First i thought about a LIKE condition to search , but i would like to have something like FIND_IN_SET or something other.

How can i achieve this in a better way ? ( For some reasons i cannot use relations on this table )

Please advice.

Example of doing a join based on FIND_IN_SET

SELECT *
FROM imap_emails
INNER JOIN customers
ON FIND_IN_SET(customers.email, imap_emails.to) > 0

You don't say how you want to narrow this down (ie, if an email uses 2 email addresses do you want both emails brought back), so can't add much at this stage.