Hi all,
Curiosity question. In the System Emails section, the default email address is the email we used when we set up the site/super-admin account. If I change this email here, does it break/change the admin login email?
The System Email Addresses dashboard page stores values in the configuration files, users won’t be touched.
So yes, it’s absolutely safe to change it.
Defiantly did not affect login, but I noticed that when I changed the default email here it quit sending emails when submitting contact forms.
You can try to look in the Logs dashboard page to see if there’s any relevant error.
Also, in the dashboard page where you can configure the SMTP server, there’s a handy page where you can check if email sending works.
No logs with errors and the spot where I can configure SMTP server test email works when I have our email listed as the default system email (that we set up the site with) but not our client’s email when I change it to theirs.
We run our email through our server (Google Workspace) and they are running a regular gmail account, so maybe it has something to do with gmail or a server-side setting?
Do you see any error when you try to send a test email via the System & Settings > Email > SMTP Method > Test Mail Settings dashboard page when you use your client’s email?
Also, please remark that Google servers are quite selective about the email address used as the sender of emails.
The only error showing in the logs are saying “Errors in reCAPTCHA validation: invalid-input-response” but it doesn’t appear that it is happening with a specific email.
Again: have you tried using the System & Settings > Email > SMTP Method > Test Mail Settings dashboard page?
If you change a sender address and then no longer receive emails from Concrete, that is very likely an SPF problem. The mailserver used to send the mails (either the webserver the site is on, or a different server if you change SMTP settings) has to be listed in the SPF-record of the domain of the email address. Otherwise most spam filters will discard the mail.
If you don’t have access to the DNS records, the best way to prevent SPF problems is to use the mail server that ‘belongs’ to that email address. Configure it in the SMTP settings and yes, as mlocati said, test it there too. 
It’s not just spf records - don’t forget big providers like Google, Yahoo etc. require DMARC and DKIM records too!
This is a handy tool to check everything is in place: Domain Health Check - Online Domain Tools - Blacklist, Email, Website, DNS - MxToolBox