The problem is found by fetchmail, which fails to fetch mail from a remote
mailserver (runing postfix+dovecot)

The console output looks like:
admin@localhost:~> fetchmail -v --ssl mail.openqa.suse
fetchmail: 6.3.26 querying mail.openqa.suse (protocol POP3) at Wed 06 Sep 2017
03:12:03 PM CST: poll started
Trying to connect to 10.67.18.229/995...connected.
fetchmail: Server certificate:
fetchmail: Issuer Organization: SUSE
fetchmail: Issuer CommonName: openQA CA
fetchmail: Subject CommonName: mail.openqa.suse
fetchmail: mail.openqa.suse key fingerprint:
4E:FD:84:BE:B5:6C:BF:B0:75:F2:F4:EF:E8:7F:50:9E
fetchmail: POP3< +OK Dovecot ready.
fetchmail: POP3> CAPA
fetchmail: POP3< +OK
fetchmail: POP3< CAPA
fetchmail: POP3< TOP
fetchmail: POP3< UIDL
fetchmail: POP3< RESP-CODES
fetchmail: POP3< PIPELINING
fetchmail: POP3< AUTH-RESP-CODE
fetchmail: POP3< USER
fetchmail: POP3< SASL PLAIN
fetchmail: POP3< .
fetchmail: POP3> USER admin
fetchmail: POP3< +OK
fetchmail: POP3> PASS *
fetchmail: POP3< +OK Logged in.
fetchmail: POP3> STAT
fetchmail: POP3< +OK 2 1446
fetchmail: POP3> LAST
fetchmail: POP3< -ERR Unknown command: LAST
fetchmail: Unknown command: LAST
fetchmail: POP3> UIDL
fetchmail: POP3< +OK
fetchmail: POP3< 1 00000016599145ec
fetchmail: POP3< 2 00000017599145ec
fetchmail: POP3< .
2 messages for admin at mail.openqa.suse (1446 octets).
fetchmail: POP3> LIST 1
fetchmail: POP3< +OK 1 741
fetchmail: POP3> TOP 1 99999999
fetchmail: POP3< +OK
reading message admin@mail.openqa.suse:1 of 2 (741 octets) 
Trying to connect to ::1/25...connected.
<fetchmail hangs here>

The system journal disclosures the real issue:
Sep 06 15:03:49 localhost postfix/smtpd[3442]: fatal: bad string length 0 < 1:
smtpd_sasl_path =
Sep 06 15:03:50 localhost postfix/master[2712]: warning: process
/usr/lib/postfix/smtpd pid 3442 exit status 1
Sep 06 15:03:50 localhost postfix/master[2712]: warning:
/usr/lib/postfix/smtpd: bad command startup -- throttling

The error can be fixed by editing the configuration file /etc/postfix/main.cf,
comment out the following lines:
smtpd_sasl_path =
smtpd_sasl_type =

Note: the new lines look like:
#smtpd_sasl_path =
#smtpd_sasl_type =