It will be the email account settings.
MS Outlook tries to be too smart for it's own good, and tries to "interpret" what the POP/IMAP and SMTP servers are going to be.
Consequently, it tries to make the server settings based on the domain name of the email address (thinking the only email addresses everyone has are ISP domain named.
So, in the event of Gmail Suite addresses, it will set the mails servers as the @
domain name part of your email address.
Why don't you change over the SMTP. All you have to dois go into your Gmail account settings (through Gmail) and change Settings (the cog wheel, top RH) > See all settings > Forwarding and POP/IMAP > down to IMAP access and select the button Enable IMAP
The change your MS Outlook settings for your GMail suite account to
imap.gmail.com + Port 993 + Connection security SSL/TLS + Authentication OAuth2
smtp.gmail.com + Port 465 + same as above
MS Outlook's attempts to do the user a favour are out of control and just plain bloody annoying - as well as GSuite accounts, I have an ISP domain name email (@ozemail.com.au) but it is not my connected ISP (TPG). Everytime I (used to) set up MS Outlook, it set the outgoing server as smtp.ozemail.com.au, which didn't exist and in any case, TPG doesn't relay outgoing email (i.e. send outgoing emails through another ISP's server .... even though TPG now owns ozemail !!!!). MS Outlook still did this every time, even though I chose the option to set up the account manually.
Needless to say, MS Outlook just languishes as an unnecessary piece of software on my drive, with the icon deleted from the Start menu. That'll teach 'em to piss me off!!
Thunderbird rulez - 1 ISP email account, 1 non-ISP domain name email account, 4 GSuite (now known as Google Workspace) email accounts, and 4 normal GMail accounts.