/usr/include/GNUstep/NGMail/ is in libsope-dev 2.2.9-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/GNUstep/NGMail/NGMBoxReader.h
/usr/include/GNUstep/NGMail/NGMail.h
/usr/include/GNUstep/NGMail/NGMailAddress.h
/usr/include/GNUstep/NGMail/NGMailAddressList.h
/usr/include/GNUstep/NGMail/NGMailAddressParser.h
/usr/include/GNUstep/NGMail/NGMailDecls.h
/usr/include/GNUstep/NGMail/NGMimeMessage.h
/usr/include/GNUstep/NGMail/NGMimeMessageGenerator.h
/usr/include/GNUstep/NGMail/NGMimeMessageParser.h
/usr/include/GNUstep/NGMail/NGPop3Client.h
/usr/include/GNUstep/NGMail/NGPop3Support.h
/usr/include/GNUstep/NGMail/NGSendMail.h
/usr/include/GNUstep/NGMail/NGSmtpClient.h
/usr/include/GNUstep/NGMail/NGSmtpSupport.h