I have ran into a problem where I cannot send email from a Mac running the latest version of Mac OS Catalina.
I even went as far as removing the iCloud and Gmail accounts and all the associated files in the user library. After adding the accounts back Mail starts to download and populate all the folders and emails.
However, whenever I try to send an email from either account it gets stuck and ends up in the “on my Mac” outbox. I checked the server settings to ensure the various folders are pointing to the ones on iCloud and Google.
Network Utility:
I have can trace and ping the outgoing mail servers to they should be reachable.
Of course, I also rebooted the machine.
I am stumped, any suggestions?
Rogier
Can you send when using the web interface to each mail system? That would eliminate server side issues. I would remove the accounts again and try with just one at a time. Try with a different email client. Can you send from iPhone and/or iPad? While you can ping the server, can you actually connect to it on the correct port? Just a few things to try.
Yes Web mail works.
No account issues.
All other devices have no problem using the same wifi network.
I had a similar-sounding issue. IMAP wasn’t syncing properly. I think I quit Mail and deleted the hidden folder .OfflineCache in /Users/Bowline/Library/Mail/ maybe further drilled down to a named IMAP folder - anyway that was the folder name. Then I restarted.
Try Googling macos mail offlinecache for details
Thanks, I even went as far as deleting the entire content of the mail folder. And I removed
all imap and smtp 
from the Keychain 
Also the related .plist from the user’s preferences.
Tomorrow I will install a 10.15.xx combo update.
I also checked for any malware and viruses using Sophos and Malwarebytes.
Were the SMTP settings associated with the email account correct? I’ve had issues with Catalina where it “spontaneously” forgot my SMTP settings for several accounts. I would have expected a popup telling me to choose an SMTP server, but that never showed up.
Yep,
I even created a new user account added the internet accounts (iCloud & Gmail) still unable to send mail from either account.
Going to nuke & pave this machine.
Not gmail accounts? (Their imap is weird)
I know that’s why I am not answering about IMAP settings. Both for iCloud and Gmail they are not visible or available in the Keychain.
This is on one of my clients computers who is far away from me so I have to remotely work on it.
I have seen this problem a couple of times before with Malware that blocks outgoing mail.
Of course I scanned for it but nothing came up.
However this is a higher value target so who knows what might be going on.
Thats why we are not taking any more chances and going to nuke & pave the laptop and rebuild from scratch.