Connection could not be established with host mail.yourdomain.com [Network is unreachable #101]

This means the outgoing port can't be connected to the host mail.yourdomain.com

The best way to check this is by checking if the port is open on the server hosting Blesta or doing a telnet on the Blesta server:
telnet mail.yourdomain.com 587

It should return something like:

root@blesta:~# telnet mail.blesta.store 587
Trying 188.165.149.229...
Connected to mail.blesta.store.
Escape character is '^]'.
220 mail.blesta.store ESMTP Postfix (Ubuntu)
EHLO mail.blesta.store
250-mail.blesta.store
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250-SMTPUTF8
250 CHUNKING

If it doesn't return a response then you need to open the outgoing port (TCP Port 587 Egress) on server. Every operating system firewall is different.

CSF: https://www.interserver.net/tips/kb/how-to-configure-csf-to-allow-outbound-smtp/
IPTABLES: https://unix.stackexchange.com/a/104957
UFW: https://www.digitalocean.com/community/questions/unable-to-allow-new-port-using-ufw?answer=20179