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: A35C2137013: to=
, relay=gmail-smtp-in.l.google.com[22.214.171.124]:25, delay=1.1, delays=0.1/0.02/0.61/0.34, dsn=5.7.1, status=bounced (host gmail-smtp-in.l.google.com[126.96.36.199] said: 550-5.7.1 [188.8.131.52 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 https://support.google.com/mail/?p=UnsolicitedMessageError 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.