Hi Brendan,
Thanks for your reply. The problem is intermittent. I am currently connected to the App store, safari is loading. Mail randomly will get email but not send. I have reset outgoing mail server multiple times. Yesterday, I reinstalled my keychain from TM backup and everything worked beautifully...for a while then reverted back to faulting.
I am using Safai 12.0 and Mojave 10.14. It is not an internet connection issue. Firefox works. On another WIFI network, I had the same problems.
I found this post https://discussions.apple.com/thread/250331441?answerId=250670877022#250670877022 which unfortunately I can't reply into now. I am downloading email as POP so I can keep it on my computer. I believe this article relates to using Gmail as IMAP. As I type at this very moment, I heard the whoosh of an email being sent! It was just a test email, no body or content only title. It has taken more than half an hour to send. The exclamation mark appears again beside the Mail inbox folder. In the Mailbox Menu under Online Status, I have the option to "Take Gmail" Offline. In the Mailbox Menu under Get New Mail, Gmail is greyed out indicating that it is offline. In Mail Connection doctor, it says "Could not connect to this POP server. Check network connection and that you have entered correct information in the Account preferences (I have). Verify it supports SSL. If not unselect Use TLS/SSL in server settings of Account Preferences.
I don't want to remove the email account and set it up again because there are 1000s of emails to download. Furthermore, I don't believe this is the issue. I think the problem lies with Mojave. As I finished typing, Mail has come in again....go figure! While that email is coming in, connection doctor still says it can't connect.
I would consider downloading Catalina but am afraid there will be multiple other issues arising out of that which I am not ready to deal with. (printer/scanner, incompatibility with excel etc). New OS has to wait.
Regards,
Ros