Translating "strange" looking addresses to Internet format

USER123 at XYZVM12
Addresses without domain (no dots in middle) are usually EARN/BITNET ones especially if they are written with capital letters or spell out @ as "at" USER123@XYZVM12.bitnet
...uunet!rix!rax!urgh
Is a typical "bang" style UUCP address meaning that once you know how to reach uunet (a very well connected UUCP node) then it ought to send it to rix which in turn should send it to rax which should be machine where the user urgh has his mailbox. It should translate to urg@rax.uucp or if rax is unknown at least in the long form: urg%rax%rix@uunet.uu.net
SOMEVMS::SOMEVAX2::SMITH
Is a DECNET path address similar to the uucp address. If SOMEVMS is a well known DECNET host (in our case known to FIGBOX gateway) then you can convert it to smith%somevax2@somevms.decnet
in%"something@cc.xyz.fi" send to them
In this case it's usually a normal internet address that is encapsulated inside the in%"" construct so you can usually take the string inside the quotes and try that. E.g. something@cc.xyz.fi in this case. This is a hack internal to VMS MAIL-11 environments used to direct the message to the Internet postoffice usually based on PMDF software. In any case if you see a abc%"address" style contruct try rereading these instructions after taking the real address from inside the quotes. In some cases it may be an EARN/BITNET address (no dots, remember?).
/G=smythe/O=abc/C=fi/
If the address has equal signs, slashes, blanks, national characters, strange acronyms and is very long it most probably is a simple user friendly standard X.400 address. To convert it to Internet you need to study a bit further like reading the X.400 ohje or trying the convertservice provided by FUNET.
Joe Smith XYZ
Is at least in Finnish context usually an Elisa address for which you need to first find the X.400 equivalent (sorry no algorithm exists) by asking the person by other means, sending mail to their mailbased directory server at address hakemisto/O=hpy/@elisa.fi with the line "X400 Firstname Lastname" in the body of the message or finally just trying to browse an outdated and LONG list of Elisa users. After that you'll need to convert the X.400 address to it's Internet equivalent keeping in mind needs for speacial characters, quoting etc. There's an Elisa ohje from FUNET that may help you. Good luck...
Telebox
A public mail service by Finnish Telecom with X.400 connection to FUNET. There's a Telebox ohjeexplaining more about it's X.400 style which is simpler to convert to Internet addresses than Elisa addresses. At least in theory...
Smail
Internal Nokia Corporation mail system similar to Elisa with similar ohje...
Compuserve: 12345,678
A large american computer conferencing etc. system. A simple note compuserve Internet gateway tells that the example should transform to: 12345.678@compuserve.com
MEMONET
A Nordic network of MEMO business mail systems running on large IBM MVS mainframes in some corporations. Information about their Internet gateway coming, try to ask for an X.400 address from the other end in the meanwhile

You could also try to look at the newsgroup comp.mail.miscfor some recent mail information posted there. Finally after finding the right address you might want to knowhow the mail is deliveredto the recipient and what may cause problems in that stage.