These are steps I take when troubleshooting a G Suite (Google Apps previously) email sending issue. At first, ten plus years ago, a Google Apps account could be set up for free for up to 50 users, then it became a paid solution, and SPAM became more of a problem so the setup process moved from an easy setup to learning everything about email routing and servers. For this particular issue, a few emails are hosted with G Suite (using G Suite default MX forwarding address) and the majority of others on a server.
Check G Suite app status for green lights on Gmail: https://www.google.com/appsstatus
Check settings on hosting: G Suite MX Forwarding per email account // G Suite had hosting email accounts when first set up but that extra physical inbox account on the hosting can hinder the reception of emails within G Suite Gmail account, so check there is no physical inbox on the hosting only the forwarding email address to the G Suite MX address (firstname.lastname@example.org) and other can be setup for separately owned domain names (within G Suite) with MX entry on a subdomain on the host, then the email forwards to the alternative MX address: email@example.com
For spam control, DKIM, SPF, and others were added on to the hosting server to show G Suite and other email systems that so-and-so servers were true representatives of the originator. These settings are picky, do it right and email works well until G Suite quietly announces (no public blog posts, found in documentation) there are more features to add to that equation.
Check email delivery using the tools in G Suite Admin without needing to change account passwords. Search for users, emails sent, times sent, were they received, did they get sent to recipient’s email address. Found under Reports > Email Log Search.
Ask user if the email problem is only them or also others in the G Suite accounts are also having problems. Narrow down where the problem exists by doing this.
Ask how they check their email. Through the web account or through a third-party email client. Find out how the emails fail, was it only sending or only receiving or both.
Sign in to G Suite through Gmail. Use domain email address for admin. Access G Suite Admin “Manage this domain” through cog wheel icon at top right in Gmail.
Check Users section, look at the user in question, check settings are correct.
Check hosting server for the email address. Is it a forwarding email or email physical inbox or both?
If emails are pulled from the host server to consumer Gmail, check settings on the host server with Gmail POP settings. May need to reset the connection by removing it and reinstalling the settings. First, send test emails to see how fast they are sent out. Check spam folder for messages that shouldn’t be there but in Inbox and mark as Not Spam. If either way doesn’t work, then check settings and reset them. Change password on the host server and make sure settings are correct on Gmail Accounts and Imports tab in Settings under cog wheel, for both Send Mail As and POP3 setup. Do not use “Treat as Alias.”
If a reset doesn’t work with POP3/SMTP (SSL, TLS, mail. or domain’s email server address, different secure ports), then use host server cPanel to find the problem. Use email diagnostics to see how the email account is set up. Check if there are account level email filters set up, which you’ll see in the diagnostics if there are. Check the routing of email. A full reset of Gmail connection to the server is needed to get new Send Mail As verification code. Changing settings won’t fix it.
Test site with external service to see if emails are being blocked in external SPAM filtering services and if SPF is set up correctly. When you enable DKIM and SPF, you blindly will see problems happen and if you’re new to it, you’ll learn lots. One way to test those values are working and that your service is not being blocked as SPAM is to use an external testing service such as https://www.mail-tester.com/. It also has a newsletter/email campaign check as its main tool.
What does an SPF record look like in your host?
v=spf1 +a +mx +ip4:50.000.000.000 +ip4:198.000.000.000 +include:externalHost.com +include:_spf.google.com +include:internalHost.com +include:mailServer.com ~all
The order matters very much if you’re using G Suite for email. Your servers need to come before the Google SPF. Some articles say the plus sign is not needed but many others say it is good practice. If v= not set right, it can put a good email into SPAM and oddly there is no alternative to this method. It’s required to be in there by Google even if it doesn’t seem to do anything turned off (v=spf1). IP4 and domain names are added to cover all “doors” for passing data between email servers and for external SPAM scanners not to mark your server as “not allowed” to send email to another server. Not allowed means they would be seen as sources of SPAM instead of partners with your email service. It’s a way to tag all your going ins/outs as trusted.
If after doing all this, you still have email problems at the external client level (such as Gmail), but your email passes through the main server both ways without issue, then take out the online middleman and use a local (your PC) third-party email program.
This is only the tip of the iceberg to troubleshoot email sending/receiving problems but if you’re learning, this information will move you in the right direction. I haven’t provided all the details but if you’re in this field of study (Routing/Switching, Networking), it’ll illuminate your path slightly. The most important lesson in troubleshooting is to have set steps that you always take from start to finish to find the problem that you want to solve.