When a mail client sends a message, that message is converted into MIME. It is the responsibility of the receiving mail server to ensure the data has been received properly, and then for the recipient’s mail client to convert the message from MIME back into a human-readable format. Also, messages encoded as base64 renders plain text unreadable in its native format. Again, it is the responsibility of the recipient’s mail client to unencode base64 into a human-readable format.
Send the same messages to another mail account you own and see if they are readable. It may simply be a configuration issue on the receiver’s end.