Correio Dovecot Matado com Sinal 15

Question

Meu Dovecot sempre falha depois de meia hora. O log de erros ( /var/log/mail.log ) diz:

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

Antes disso, meu servidor de e-mail é executado bem. Depois disso, preciso reiniciar o serviço dovecot. O que está matando o pombal?