in /etc/dovecot/conf.d/10-ssl.conf add
ssl_cert = </etc/path/to/your/cert.pem
ssl_key = </etc/path/to/your/key.pem
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in /etc/postfix/main.cf add
smtpd_tls_cert_file = /etc/path/to/your/cert.pem
smtpd_tls_key_file = /etc/path/to/your/key.pem
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