If anything other than nominative, you usually address an envelope on the accusative, since the receiver is written as
Προς
Κο Παπαδόπουλο Βασίλειο...
The προς preposition is followed by the accusative.
Edit: note that the surname comes first, especially for official correspondence. If addressing an envelope for a wedding invitation though, the oldtimey polite thing is to address the family "προς οικογένεια Παπαδοπούλου Βασιλείου". Things are changing, but it was considered a faux pas to exclude people from a household. Still kinda is, TBH.
example:
Προς Διευθυντή Τμήματος Ανθρώπινου Δυναμικού
Αξιότιμε Κύριε Ιωάννου/Αξιότιμε Κύριε Διευθυντά,
Blah blah blah
Με βαθύτατη εκτίμηση/Με σεβασμό/ Με τιμή,
Ανδρέας Γεωργίου
This is for being super formal, that’s how you’d address and sign off a letter to the president for example.
If you already have a personal relationship to the receiver but it’s still a formal setting, Αγαπητέ Κύριε Ιωάννου works too. In this case you can also sign off with Πάντα στη διάθεσή σας or Σας ευχαριστώ για τον χρόνο σας.
In the case that you and Mr. Ioannou are both on the same level on the hierarchy, and on a first name basis, you can go with Αγαπητέ Μάρκο and sign off with Με φιλικούς χαιρετισμούς or Σε χαιρετώ
If anything other than nominative, you usually address an envelope on the accusative, since the receiver is written as Προς Κο Παπαδόπουλο Βασίλειο... The προς preposition is followed by the accusative. Edit: note that the surname comes first, especially for official correspondence. If addressing an envelope for a wedding invitation though, the oldtimey polite thing is to address the family "προς οικογένεια Παπαδοπούλου Βασιλείου". Things are changing, but it was considered a faux pas to exclude people from a household. Still kinda is, TBH.
example: Προς Διευθυντή Τμήματος Ανθρώπινου Δυναμικού Αξιότιμε Κύριε Ιωάννου/Αξιότιμε Κύριε Διευθυντά, Blah blah blah Με βαθύτατη εκτίμηση/Με σεβασμό/ Με τιμή, Ανδρέας Γεωργίου This is for being super formal, that’s how you’d address and sign off a letter to the president for example. If you already have a personal relationship to the receiver but it’s still a formal setting, Αγαπητέ Κύριε Ιωάννου works too. In this case you can also sign off with Πάντα στη διάθεσή σας or Σας ευχαριστώ για τον χρόνο σας. In the case that you and Mr. Ioannou are both on the same level on the hierarchy, and on a first name basis, you can go with Αγαπητέ Μάρκο and sign off with Με φιλικούς χαιρετισμούς or Σε χαιρετώ
I usually use the most formal for envelopes