In the last couple of years many large email providers have started to refuse emails coming from certain IP addresses or according to certain other criteria. It is difficult to know for what reason certain emails are refused access with the server answer:
....refused to talk to me: 554....
The list of these emails destinations(which is growing by the day) is for example:
aol.com
verizon.net
att.net
bellsouth.net
aim.com
yahoo.com
gmx.at
gmx.net
gmx.de
online.de
heikoloechel.de
freenet.de
aol.de
snafu.de
web.de
Therefore one temporary solution(till I can fix the cause) is to relay the problematic emails via a relay email server which is more accepted.
Here is the Postfix settings that allows me to do that:
Added the following entry in /etc/postfix/main.cf
transport_maps = hash:/etc/postfix/transport
And in the file /etc/postfix/transport
aol.com relay:[my.relay.com]:25
verizon.net relay:[my.relay.com]:25
att.net relay:[my.relay.com]:25
bellsouth.net relay:[my.relay.com]:25
aim.com relay:[my.relay.com]:25
yahoo.com relay:[my.relay.com]:25
gmx.de relay:[my.relay.com]:25
gmx.at relay:[my.relay.com]:25
online.de relay:[my.relay.com]:25
heikoloechel.de relay:[my.relay.com]:25
freenet.de relay:[my.relay.com]:25
aol.de relay:[my.relay.com]:25
snafu.de relay:[my.relay.com]:25
gmx.net relay:[my.relay.com]:25
web.de relay:[my.relay.com]:25
Note: Here obviously you need to replace my.relay.com for your own relay server.
Hash the file for postfix:
postmap /etc/postfix/transport
And reload Postfix:
service postfix reload