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This is unrelated to our software. It's even unrelated to Joomla itself. It tells you that your mail server declined the From email address you have set up.

I feel obliged to provide a clarification for two points I considered obvious in my previous reply, thus decided to not reference in an attempt to not come across as condescending or worse.

We do not use “the Joomla mail config”. Akeeba Backup calls the Joomla Mail API and hands it the recipient address and message content. Joomla itself — not our software — creates the SMTP connection, authenticates, and transmits the message. We have no involvement in how the email is sent, which port is used, or how the connection is established. The error you are seeing originates inside Joomla’s mailer (PHPMailer), which is reporting a rejection received from your SMTP server.

“MAIL FROM command failed” is an SMTP protocol error, and it is coming directly from your mail server. For your server to return that error, the TCP connection to it over port 26 must have already been established successfully and SMTP negotiation must have already begun — otherwise the error would read “Connection refused” or “Connection timed out”, not “MAIL FROM command failed”. The SMTP server accepted the connection from your cron process, proceeded through the greeting and EHLO exchange, and then explicitly rejected the sender address ([email protected]) when the MAIL FROM command was issued. That is a decision made by your mail server, not by our software, not by Joomla, and not by your host’s firewall.

As for why the “Send Test Email” succeeds: one possibility, though admittedly unlikely, is that Joomla’s Mail Templates allow each template to define its own sender address independently of the Global Configuration. If the backup notification template has a different sender configured, that address — rather than the one you tested — would be presented to your mail server in the MAIL FROM command, and your mail provider may not have authorised it to send.

I am afraid that we are unable to help further, but this is genuinely outside the scope of what we support. We make a call to Joomla’s mail API; what happens from that point onwards is entirely between Joomla, your server, and your mail provider — none of which we have any visibility into or control over. It does rather sound as though Digital Ocean are the right people to be speaking with, and we do hope they get it sorted for you promptly.