为什么老是连接超时了，过一会就不行了

May 20 20:55:45 mail postfix/anvil[4640]: statistics: max connection rate 5/60s for (submission:60.179.5.148) at May 20 20:52:25
May 20 20:55:45 mail postfix/anvil[4640]: statistics: max connection count 2 for (submission:60.179.5.148) at May 20 20:51:44
May 20 20:55:45 mail postfix/anvil[4640]: statistics: max cache size 1 at May 20 20:51:44
max connection rate 5/60s for (submission:60.179.5.148) aMay 20 20:58:34 mail postfix/qmgr[1932]: 095026191A: from=<test@eqnvip.cn>, size=1697, nrcpt=1 (queue active)
May 20 20:59:04 mail postfix/smtp[4724]: connect to mx3.qq.com[183.57.48.35]:25: Connection timed out
May 20 20:59:34 mail postfix/smtp[4724]: connect to mx2.qq.com[59.37.97.124]:25: Connection timed out
May 20 21:00:04 mail postfix/smtp[4724]: connect to mx2.qq.com[14.17.41.170]:25: Connection timed out
May 20 21:00:34 mail postfix/smtp[4724]: connect to mx1.qq.com[183.57.48.35]:25: Connection timed out
May 20 21:00:34 mail postfix/smtp[4724]: 095026191A: to=<123561326@qq.com>, relay=none, delay=21230, delays=21110/0.01/120/0, dsn=4.4.1, status=deferred (connect to mx1.qq.com[183.57.48.35]:25: Connection timed out)