gmail 第一次使用imap，在後臺設定開啟IMAP功能，可能會遇到接收不到郵件，報錯：

Couldn't open stream {imap.gmail.com:993/imap/ssl/novalidate-cert}INBOX      Cannot connect to server: Can not authenticate to IMAP server: [ALERT]

這是安全問題，需要驗證，您登陸郵箱，然後訪問http://accounts.google.com/DisplayUnlockCaptcha

然後點選繼續，就可以了

英文參考：

I clicked the link (accounts.google.com/DisplayUnlockCaptcha) from my PC. It replied that my account was unlocked. Then when I tried the program again from AWS instance, it worked and now I can read gmail emails. This had to be locked from the beginning, but I don't know how my other servers (Netfirms and my PC) were able to read, but not AWS. One person guessed, because it was a new IP address that gmail blocked it right out of the gate.