Since many today have an issue with the gmail blocking them in case of the spam, here is a short info about it.

If you see in your postfix log file /var/log/maillog messages like:

Mar 29 08:13:02 cwp-mailserver postfix/smtp[12934]: A35C2137013: to=,[]:25, delay=1.1, delays=0.1/0.02/0.61/0.34, dsn=5.7.1, status=bounced (host[] said: 550-5.7.1 [      12] Our system has detected that this message is 550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 550-5.7.1 this message has been blocked. Please visit 550-5.7.1 550 5.7.1  for more information. hc31si12141407ejc.524 - gsmtp (in reply to end of DATA command))

This can mean in some cases that gmail blocked your domain and/or IP address.
In a case of the blocked domain when you resolve the issue with spam they could continue to block all the emails for the next 30 days.