Needed to move my mail-account from one sogo-installation to a new one (both in the context of mailcow). Both sogo versions are the same (4.3.2). Exported my addressbook as ldif and wanted to import it on the new one, got the message 0 entries imported. Tried different times to export and import, each time same result. Took a look on the exported ldif, all contacts were there. Removed the first one and retried to import, this time some of the contacts were imported, but not all. Examined the differences and found out the following:

objectClass: top
objectClass: inetOrgPerson
objectClass: mozillaAbPersonAlpha
sn: Somelastname
mobile: +49 123 1234567
givenname: Somefirstname

dn: mail=somename2@somedomain.com
objectClass: top
objectClass: inetOrgPerson
objectClass: mozillaAbPersonAlpha
mail: somename2@somedomain.com
sn: Somelastname2
mobile: +49 321 7654321
givenname: Somefirstname2

objectClass: top
objectClass: inetOrgPerson
objectClass: mozillaAbPersonAlpha
sn: Somelastname3
homephone: +49 987 9876543
mobile: +49 123 7654321
givenname: Somefirstname3

The ldif starting like this did not import anything, removing the first entry imported all contacts with a mail-address. After adding something like dn: mail=no@mail.com as first line to every contact without a mail-address, I could import all contacts. In sogo-addressbook this mailaddress was not added to the contacts.