我的 Dovecot 总是在大约半小时后崩溃。错误日志 ( /var/log/mail.log) 表示：
Sep 24 12:05:07 yetzner dovecot: imap-login: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Sep 24 12:05:07 yetzner dovecot: message repeated 2 times: [ imap-login: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)]
Sep 24 12:05:07 yetzner dovecot: auth-worker(32132): Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Sep 24 12:05:07 yetzner dovecot: imap: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Sep 24 12:05:07 yetzner dovecot: imap: Server shutting down. in=150 out=591
Sep 24 12:05:07 yetzner dovecot: imap: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Sep 24 12:05:07 yetzner dovecot: imap: Server shutting down. in=889 out=3101
Sep 24 12:05:07 yetzner dovecot: imap: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Sep 24 12:05:07 yetzner dovecot: imap: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Sep 24 12:05:07 yetzner dovecot: imap: Server shutting down. in=9446 out=201067
Sep 24 12:05:07 yetzner dovecot: imap: Server shutting down. in=10705 out=1969769
Sep 24 12:05:07 yetzner dovecot: imap: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Sep 24 12:05:07 yetzner dovecot: imap: Server shutting down. in=10109 out=458737
Sep 24 12:05:07 yetzner dovecot: imap: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Sep 24 12:05:07 yetzner dovecot: imap: Server shutting down. in=9041 out=154798
Sep 24 12:05:07 yetzner dovecot: imap: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Sep 24 12:05:07 yetzner dovecot: imap: Server shutting down. in=11253 out=462703
在此之前，我的邮件服务器运行良好。之后，我必须重新启动 dovecot 服务。什么是杀死鸽子？