When you created the retain SQL DB did you use utf8mb4 encoding ?

CREATE DATABASE retain DEFAULT CHARACTER SET utf8mb4 DEFAULT COLLATE utf8mb4_unicode_ci;

I used the following line to create the database. 
CREATE DATABASE retain WITH OWNER = retainuser ENCODING = 'UTF8' TABLESPACE = pg_default CONNECTION LIMIT = -1;

This is the exact line specified in the documentation (Creating and Maintaining an SQL Database - OpenText Retain CE 24.2: Installation and Upgrade (novell.com)), although I have postgres 12.14 running in a docker container.

Is there any way to change the encoding of the database once the emails have already been archived?

I do not know if this is the root issue for your case.  I just know that for MySQL, MariaDB we were told to start using the new utf8mb4 encoding over the older utf8 when we deploy Retain.

Looking trough postgres documentation it does not look like postgres supports different variations of utf8 encoding. On the other hand, MySQL and MariaDB do.

This is also reflected in Retain documentation linked to earlier. If you see the CREATE DATABASE line for the latter two databases it does state utf8mb4 encoding.

In conclusion, either postgres is not the best option for Retain because it does not provide encoding capabilities required or there is another fix to this problem.

I think your creation statement is okay. I have some customers using postgresql for Retain in the background. I am not aware that someone complains about characters.

Are you able to open case? I know you are in the evaluation phase ... But they have some very good support guys around database management. Especially Georg.

No, sadly we only get demonstration&enablement licences and it it not possible to open a service request.

Hello Nik, if this is a demonstration environment you are using Retain with, i might have some steps you can use to sanitize your broken system. 

As you cant open a support case, could you PN me your e-mail so i can send you the necessary steps?

I'm using PostgreSQL and UTF8 is OK for Czech language. But sometimes we get emails especially from Exchange world, which had strange encoding and there is a problem with such characters.