You can add a mail domain for your hostname cp.example.com, add the right DKIM, DMARC and SPF records and add an account postmaster with root and noreply as alias an you shouldn’t have issues.
I’m reading of a way using ssmtp where I can add the credentials I’ve made for an inbox in my mail domain. Does it sound good?
If you want to test it, go ahead but I think it’s easier to add the hostname as mail domain in Hestia.
But how do I indicate to the crontab (or the system in general) to use those credentials? 
This is the part I’m missing
I’ve never done that with crontab, so I’m not sure whether it’s possible… at least not easily. Some Ubuntu versions allow the MAILFROM="" variable in crontab, but you’d need to add it manually (Debian doesn’t support this variable).
As I mentioned, adding the hostname as the mail domain will make your life easier. 
I should have listen to you and slow down, exim isn’t working anymore since I’ve tried to follow the guide I found using ssmtp. I’ll try to install it from scratch
Is there a way to reinstall exim from HestiaCP? The /etc/exim4/exim4.conf.template file got reset unfortunately
I would say that you can restore it from your backup… 
The way to restore it will depend on the services you are using… clamav, spamassassin, sieve… I’ve wrote several posts in this forums regarding it. I’ve no time now but if you can’t solve it, let me know what are the services you are using and i’ll tell you the way to restore Exim’s conf later.
Since I moved to Oracle I never performed a restore and unfortunately I found out later that their support takes ages to reply so I would use a backup only in extreme cases 
I was able to get the default conf file from GitHub but I think that the way I’ve reinstalled exim is not the default type of install that HestiaCP does as it doesn’t accept the default conf file. It won’t restart when using it:
authenticator dovecot_plain: cannot find authenticator driver “dovecot”
I think I was able to bring that back up, but Roundcube keeps saying Login failed even if the credentials are correct
I’m back.
Great! 
Regarding roundcube:
Show me the output of telnet localhost 143
All good, I believe, but please, if you can, confirm that I’m not missing anything.
By checking dovecot logs I was able to see that exim4 was missing the /domains folder as well as passwd. Luckily I made a copy of that folder before reinstalling so I got them from there, current owner is root:root. The login now works as well as receive/sending emails. Is there anything else I should double check?
Tell me what’s your Exim version and whether you are using Sieve:
dpkg -l | grep -E 'exim|sieve'
Also, show the output of these commands:
ls -la /etc/exim4/
grep -Ev '^$|^#' /etc/exim4/update-exim4.conf.conf
Thank you!
ii exim4 4.93-13ubuntu1.12 all metapackage to ease Exim MTA (v4) installation
ii exim4-base 4.93-13ubuntu1.12 arm64 support files for all Exim MTA (v4) packages
ii exim4-config 4.93-13ubuntu1.12 all configuration for the Exim MTA (v4)
ii exim4-daemon-heavy 4.93-13ubuntu1.12 arm64 Exim MTA (v4) daemon with extended features, including exiscan-acl
rc exim4-daemon-light 4.93-13ubuntu1.12 arm64 lightweight Exim MTA (v4) daemon
root@1:~# ls -la /etc/exim4/
total 96
drwxr-xr-x 6 root root 4096 Dec 14 21:26 .
drwxr-xr-x 136 root root 12288 Dec 14 18:26 ..
drwxr-xr-x 9 root root 4096 Dec 14 18:26 conf.d
-rw-r--r-- 1 root root 32 Dec 14 21:26 dnsbl.conf
drwxr-xr-x 2 root root 4096 Dec 14 21:26 domains
drwxr-xr-x 4 root root 4096 Dec 14 21:24 exim4.backup
-rw-r--r-- 1 root root 17188 Dec 14 21:19 exim4.conf.template
-rw-r--r-- 1 root root 17175 Dec 14 21:19 exim4.conf.template.backup2
-rw-r--r-- 1 root root 3 Dec 14 21:26 limit.conf
drwxr-xr-x 2 root root 4096 Dec 14 21:26 passwd
-rw-r----- 1 root Debian-exim 204 Jul 30 19:25 passwd.client
-rw-r--r-- 1 root root 0 Dec 14 21:26 spam-blocks.conf
-rw-r----- 1 root root 17 Dec 14 21:26 srs.conf
-rw-r--r-- 1 root root 442 Dec 14 21:26 system.filter
-rw-r--r-- 1 root root 1030 Dec 14 21:16 update-exim4.conf.conf
-rw-r--r-- 1 root root 0 Dec 14 21:26 white-blocks.conf
root@1:~# grep -Ev '^$|^#' /etc/exim4/update-exim4.conf.conf
dc_eximconfig_configtype='internet'
dc_other_hostnames='localbizone-x'
dc_local_interfaces='0.0.0.0'
dc_readhost=''
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost=''
CFILEMODE='644'
dc_use_split_config='false'
dc_hide_mailname=''
dc_mailname_in_oh='true'
dc_localdelivery='mail_spool'
That should be:
dc_eximconfig_configtype='local'
dc_other_hostnames='HereYourServerHostname'
dc_local_interfaces='127.0.0.1 ; ::1'
You are not using Sieve and your Exim version is a bit old so you are not using SRS too, only Spamassassin and Clamav so this script should build the right conf for your system.
#!/usr/bin/env bash
HESTIA_INSTALL_DIR="/usr/local/hestia/install/deb"
eximdir="/etc/exim4"
backup="/root/backup_etc_exim4_$(date +'%Y-%m-%d_%H$M$S')/etc"
mkdir -p "$backup"
cp -rf "$eximdir" "$backup"
cp -f $HESTIA_INSTALL_DIR/exim/exim4.conf.template "$eximdir"/exim4.conf.template
sed -Ei "s/^#SPAM/SPAM/" "$eximdir"/exim4.conf.template
sed -Ei "s/^#CLAMD/CLAMD/" "$eximdir"/exim4.conf.template
chmod 640 "$eximdir"/exim4.conf.template
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Let me edit that, by hostname what does it refers to? The domain used by HestiaCP?
hostname -f
All done, should I restart exim4?
Yes, you should 
It seems that all is well, finally 
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