I've executed:
# grep -i 'greylist' /var/log/mail.log
but I only get this lines:
Mar 26 11:49:58 smtp postfix/smtp[26059]: 3030D6801AF: host svrm03.ibernet.net[81.92.136.12] said: 450 <unknown[82.194.94.xx]>: Client host rejected: Greylisted (in reply to RCPT TO command)
Mar 26 11:49:58 smtp postfix/smtp[26059]: 3030D6801AF: to=<xxxx.xxx@xxxxxx.com>, relay=svmb02.ibernet.net[81.92.136.11]:25, delay=0.53, delays=0.01/0.01/0.38/0.12, dsn=4.0.0, status=deferred (host svmb02.ibernet.net[81.92.136.11] said: 450 <xxxx.xxx@xxxxxxx.com>: Recipient address rejected: Greylisted (in reply to RCPT TO command))
Mar 26 13:42:19 smtp postfix/smtp[1811]: 33B2C6801A9: to=<yyyy.yyy@yyyyy.com>, relay=mail.xxxxxx.com[195.53.213.xxx]:25, delay=0.06, delays=0.01/0/0.02/0.02, dsn=4.0.0, status=deferred (host mail.xxxxx.com[195.53.213.xxx] said: 451 Greylisting enabled, try again in 5 minutes (in reply to RCPT TO command))
Mar 27 10:41:26 smtp postfix/smtp[17486]: 6C2B2680198: host xx.xxxx.xx.uk[212.53.64.xx] said: 451 82.194.94.xx - You have been greylisted, please try later (in reply to RCPT TO command)
Mar 27 11:20:39 smtp postfix/smtp[21665]: D2CBD6801C6: to=<xxxx@xxxx.es>, relay=xxxx.xxx.es[93.92.232.xx]:25, delay=4.1, delays=0.02/0/0.21/3.9, dsn=4.2.0, status=deferred (host xxxx.xxx.es[93.92.232.xx] said: 450 4.2.0 <xxxx@xxx.es>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/xxxx.es.html (in reply to RCPT TO command))
Mar 27 11:23:11 smtp postfix/smtp[21866]: 2C7206801C7: host easymx2.easily.co.uk[62.128.158.225] said: 451 82.194.94.xx - You have been greylisted, please try later (in reply to RCPT TO command)
Mar 27 11:23:12 smtp postfix/smtp[21866]: 2C7206801C7: to=<xxxxx@xxxxxx.co.uk>, relay=rhea.easily.co.uk[212.53.64.82]:25, delay=1.1, delays=0.05/0.01/0.82/0.2, dsn=4.0.0, status=deferred (host rhea.easily.co.uk[212.53.64.82] said: 451 82.194.94.xx - You have been greylisted, please try later (in reply to RCPT TO command))
Mar 27 11:53:36 smtp postfix/smtp[2423]: 49A7568017D: host rhea.easily.co.uk[212.53.64.82] said: 451 82.194.94.xx - You have been greylisted, please try later (in reply to RCPT TO command)
Mar 27 16:39:44 smtp postfix/smtp[15794]: 6F98E6801DB: host spool.mail.gandi.net[217.70.184.6] said: 450 4.7.1 <xxxx@xxxxxxx.es>: Recipient address rejected: Greylisted for 299 seconds (in reply to RCPT TO command)
Mar 27 16:39:45 smtp postfix/smtp[15794]: 6F98E6801DB: to=<xxxx@xxxxxxx.es>, relay=fb.mail.gandi.net[217.70.184.162]:25, delay=2, delays=1/0/0.56/0.48, dsn=4.7.1, status=deferred (host fb.mail.gandi.net[217.70.184.162] said: 450 4.7.1 <xxxx@xxxxxxx.es>: Recipient address rejected: Greylisted for 298 seconds (in reply to RCPT TO command))
Mar 28 14:10:09 smtp postfix/smtp[10355]: BC55B6801A2: to=<xxx@xxxxxxxx.com>, relay=mail.nivas.hr[82.94.255.91]:25, delay=1.1, delays=0.06/0.01/0.31/0.77, dsn=4.2.0, status=deferred (host mail.nivas.hr[82.94.255.91] said: 450 4.2.0 <xxx@xxxxxx.com>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/xxxxxxxx.com.html (in reply to RCPT TO command))
Mar 28 16:40:51 smtp postfix/smtp[26295]: 4CC3B6801AB: host mx1.infoself.com[89.140.177.141] said: 450 4.2.0 <xxxxxxx@xxxxxx.es>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/xxxxxxxx.es.html (in reply to RCPT TO command)
Mar 28 16:40:51 smtp postfix/smtp[26295]: 4CC3B6801AB: to=<xxxxxx@xxxxxx.es>, relay=mx2.infoself.com[89.140.177.142]:25, delay=0.53, delays=0.01/0.01/0.33/0.19, dsn=4.2.0, status=deferred (host mx2.infoself.com[89.140.177.142] said: 450 4.2.0 <xxxxxx@xxxxxx.es>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/xxxxxxx.es.html (in reply to RCPT TO command))
Mar 29 11:14:07 smtp postfix/smtp[10162]: 19AFA6801B6: to=<xxxx@xxxxx.biz>, relay=mail.s70.biz[80.80.88.71]:25, delay=33, delays=1/0.01/16/16, dsn=4.0.0, status=deferred (host mail.s70.biz[80.80.88.71] said: 451 Greylisting enabled, try again in 2 minutes (in reply to RCPT TO command))
It seems that greylisting only works for outgoing mail.
Still getting spam in the user's mailbox.