I have set up a mail server using postfix and dovecot: mail.milovanovic.nl. It works well enough (sending and receiving), except for the fact that I cannot receive mail from certain services, like Reddit and Firefox (confirmation mail to obtain a Firefox account). It is receiving mail from most services so far without problems.
My question is, what causes this and how can I fix it?
I don't know what other information would be relevant to include, apologies.
EDIT: most recent part of my mail.log
Dec 30 12:23:44 milovanovic postfix/smtpd[20815]: connect from unknown[45.142.120.166]
Dec 30 12:23:44 milovanovic postfix/smtpd[20458]: warning: unknown[141.98.10.136]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
Dec 30 12:23:44 milovanovic postfix/smtpd[20458]: disconnect from unknown[141.98.10.136] ehlo=1 auth=0/1 quit=1 commands=2/3
Dec 30 12:23:44 milovanovic postfix/smtpd[21309]: disconnect from unknown[212.70.149.85] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Dec 30 12:23:48 milovanovic postfix/smtpd[20504]: connect from unknown[212.70.149.54]
Dec 30 12:23:49 milovanovic postfix/smtpd[20465]: connect from unknown[45.142.120.166]
Dec 30 12:23:50 milovanovic postfix/smtpd[20336]: warning: unknown[45.142.120.166]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
Dec 30 12:23:50 milovanovic postfix/smtpd[20336]: disconnect from unknown[45.142.120.166] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Dec 30 12:23:51 milovanovic postfix/smtpd[21804]: warning: unknown[45.142.120.166]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
Dec 30 12:23:51 milovanovic postfix/smtpd[21804]: disconnect from unknown[45.142.120.166] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Dec 30 12:23:51 milovanovic postfix/smtpd[20553]: connect from unknown[212.70.149.85]
Dec 30 12:23:54 milovanovic postfix/smtpd[21894]: connect from unknown[45.142.120.166]
Dec 30 12:23:57 milovanovic dovecot: auth: Warning: auth client 0 disconnected with 1 pending requests: Connection reset by peer
Dec 30 12:23:58 milovanovic postfix/anvil[12765]: statistics: max connection rate 14/60s for (smtp:45.142.120.166) at Dec 30 12:19:16
Dec 30 12:23:58 milovanovic postfix/anvil[12765]: statistics: max connection count 4 for (smtp:45.142.120.166) at Dec 30 12:14:18
Dec 30 12:23:58 milovanovic postfix/anvil[12765]: statistics: max cache size 15 at Dec 30 12:19:43
Dec 30 12:23:58 milovanovic postfix/smtpd[21309]: connect from unknown[45.142.120.166]
Dec 30 12:23:59 milovanovic postfix/smtpd[20815]: warning: unknown[45.142.120.166]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
Dec 30 12:23:59 milovanovic postfix/smtpd[20504]: warning: unknown[212.70.149.54]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
Dec 30 12:23:59 milovanovic postfix/smtpd[20815]: disconnect from unknown[45.142.120.166] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Dec 30 12:23:59 milovanovic postfix/smtpd[20465]: warning: unknown[45.142.120.166]: SASL LOGIN authentication failed: Connection lost to authentication server
Dec 30 12:23:59 milovanovic postfix/smtpd[20465]: disconnect from unknown[45.142.120.166] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Dec 30 12:24:00 milovanovic postfix/smtpd[20504]: disconnect from unknown[212.70.149.54] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Dec 30 12:24:03 milovanovic postfix/smtpd[21804]: connect from unknown[45.142.120.166]
Dec 30 12:24:05 milovanovic postfix/smtpd[20553]: warning: unknown[212.70.149.85]: SASL LOGIN authentication failed: UGFzc3d
I found out what my issue was some time ago. I blindly followed a tutorial for DMARC, and it turned out that in that tutorial they simply set the policy to reject mails that do not authenticate. Hence, the Reddit and Firefox mails got rejected.
I changed the policy to not reject them, and voila, no more rejections.