==== Required information ====
- iRedMail version (check /etc/iredmail-release): 0.9.5-1
- Linux/BSD distribution name and version: centos 6.5
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): mysql
- Web server (Apache or Nginx): nginx
- Manage mail accounts with iRedAdmin-Pro? yes
- Related log if you're reporting an issue:
现状描述:
原邮箱域名为a.com 现在以域别名的方式为a.com 添加b.com，现在有用户xx@b.com  需要以账户xx@b.com 登陆 邮箱系统

当收件为pop是  可以验证密码通过，日志如下（ dovecot.log）：
Oct 19 14:05:01 pop3-login: Info: Aborted login (auth failed, 1 attempts in 2 secs): user=<xx@b.com>, method=PLAIN, rip=121.227.47.159, lip=10.168.119.141, TLS, session=<64HIljE/TAB54y+f>

Oct 19 14:05:05 pop3-login: Info: Login: user=<xx@a.com>, method=PLAIN, rip=121.227.47.159, lip=10.168.119.141, mpid=18564, TLS, session=<tFHpljE/TQB54y+f>
Oct 19 14:05:05 pop3(zcl@moojnn.com): Info: Disconnected: Logged out top=0/0, retr=0/0, del=0/354, size=29233644

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观察先以 b域名验证失败后 又以a域名验证？

当收件为imap时，验证通不过，日志如下（ dovecot.log）：
Oct 19 14:20:04 imap-login: Info: Disconnected (auth failed, 1 attempts in 8 secs): user=<xx@b.com>, method=PLAIN, rip=121.227.47.159, lip=10.168.119.141, TLS, session=<m009zDE/CQB54y+f>

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当发件为smtp 发送时，发现也是先认证失败后在以a.com 验证（maillog）。
Oct 19 14:21:52 mail postfix/submission/smtpd[16404]: warning: unknown[121.227.47.159]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
Oct 19 14:21:56 mail postfix/submission/smtpd[16404]: 30EEAC0EC9: client=unknown[121.227.47.159], sasl_method=LOGIN, sasl_username=xx@a.com

发现 iredmail 的用户认证时通过 dovecot 完成的。 能否修改 dovecot 来完成别名域验证？