According to RFC 5322, an email may only contain 7-bit ASCII characters. There are only 128 characters available, which today may be limiting. Current email clients use a different code for characters which do not appear in the 7-bit list, such as "ü". Codes vary and may lead to problems if the sender and recipients use different codes. You will find a detailed description of the problem here (in German).
Problems may arise in particular when senders use a type of code that recipients may not decode. Companies such as Microsoft and Apple work hard to optimize their email clients and make sure that they can decode as many codes as possible.
The 4D database is not a dedicated email client and does not do well with all the various codes. Emails may thus not always be decoded properly and you may sometimes not see all special characters.
Coding Your Own Emails
While you cannot control which codes senders use, you have control over the code that you wish to use. Make sure that it is compatible with the 4D database.
Make sure that your email client uses the right format:
Apple Mail uses iso-8858-8-i. There are no issues with this format in 4D.
All versions of Outlook are different. If you're having problems with archiving your own emails, make sure that you are using the code "Western Europe (ISO)".