How can I make a foreign-key case-insensitive in Postgresql?

I have a Mariab-database using collation utf8bm4_general_ci and I am migrating to Postgresql. Some tables are foreign keyed using case-insensitive strings (e-mails sourced from different systems). When I use pgloader to migrate, I get an error message stating that some keys are not present. I can update the source table to be able to do the migration, but the error will present itself after a while if I do the switch. What can I do to fix this other than rewriting and debugging all systems to make sure e-mails are lowercased?