Exim - ошибка gnutls handshake - A TLS fatal alert has been received

Столкнулись с ошибкой на отправляющей стороне (SMTP хост mail.site.net, Port 465 и SSL, ОС стоит CentOS 7):

Connection could not be established with host mail.site.net :stream_socket_client(): SSL operation failed with code 1.
OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed Log data: ++
Starting Swift_SmtpTransport !! Connection could not be established with host mail.site.net :stream_socket_client():
SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed  (code: 0) 
++ Starting Swift_SmtpTransport !! Connection could not be established with host mail.site.net :stream_socket_client():
SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed (code: 0)

на сервере (Ubuntu 20.04) через который делается отправка в логах появился ошибка:

TLS error on connection from [XX.XX.XX.XX] (gnutls_handshake): A TLS fatal alert has been received.

Решение: 1) Обновить все ПО на серверах:

# yum update -y
# apt-get update && apt-get upgrade -y

2) На сервере с Ubuntu установить gnutls-bin и перезапустить exim4:

# apt-get install gnutls-bin -y
# systemctl restart exim4