Olá,
Vamos deixar aqui registada a provada da falha de autenticação dos seus emails que provoca o bloqueio do IP.
Pode ver a mensagem "auth failed" que significa autenticação falhada. Neste log são dois emails que estão a falhar. O servidor está com horário de Lisboa-Portugal.
Feb 21 15:35:48 s8 dovecot[28119]: imap-login: Disconnected (auth failed, 1 attempts in 2 secs): user=<contato@tenharenda.net>, method=PLAIN, rip=127.0.0.1, lip=127.0.0
.1, secured, session=<byd/xhefdqZ/AAAB>
Feb 21 15:36:24 s8 dovecot[28119]: auth: passwd-file(
contato@tenharenda.net,127.0.0.1,<OnHByBefyqd/AAAB>): Password mismatch
Feb 21 15:36:26 s8 dovecot[28119]: imap-login: Disconnected (auth failed, 1 attempts in 2 secs): user=<contato@tenharenda.net>, method=PLAIN, rip=127.0.0.1, lip=127.0.0
.1, secured, session=<OnHByBefyqd/AAAB>
Feb 21 15:39:34 s8 dovecot[28119]: auth: passwd-file(
consultoria@tenharenda.net,127.0.0.1,<mI8K1BefkK5/AAAB>): Password mismatch
Feb 21 15:39:36 s8 dovecot[28119]: imap-login: Disconnected (auth failed, 1 attempts in 2 secs): user=<consultoria@tenharenda.net>, method=PLAIN, rip=127.0.0.1, lip=127
.0.0.1, secured, session=<mI8K1BefkK5/AAAB>
Feb 21 15:39:51 s8 dovecot[28119]: auth: passwd-file(
consultoria@tenharenda.net,127.0.0.1,<zq8O1RefOq9/AAAB>): Password mismatch
Feb 21 15:39:53 s8 dovecot[28119]: imap-login: Disconnected (auth failed, 1 attempts in 2 secs): user=<consultoria@tenharenda.net>, method=PLAIN, rip=127.0.0.1, lip=127
.0.0.1, secured, session=<zq8O1RefOq9/AAAB>
Feb 21 15:43:44 s8 dovecot[28119]: auth: passwd-file(
consultoria@tenharenda.net,127.0.0.1,<Tzz04hef/Ld/AAAB>): Password mismatch
Feb 21 15:43:46 s8 dovecot[28119]: imap-login: Disconnected (auth failed, 1 attempts in 2 secs): user=<consultoria@tenharenda.net>, method=PLAIN, rip=127.0.0.1, lip=127
.0.0.1, secured, session=<Tzz04hef/Ld/AAAB>