I have a 2018 15" MacBook Pro running macOS Mojave 10.14.2. I am unable to send emails from any account (be it Yahoo, Hotmail, AOL or others) using any app (tried the default Mail app, Canary Mail, Spark and Airmail). I am, however, able to receive emails without any problems.
Whenever I try to send an email, it ends up Outbox, and I get a prompt to specify outgoing server, which I do for the specific mail servers according to the official requirements on their websites. Most of the times, I cannot even add the outgoing server into the SMTP Server List because after adding it and clicking OK, it doesn't remain therein, it just disappears. Those few times I somehow managed to add an outgoing server into the SMTP Server List, I still was not able to send emails even though in the Connection Doctor the connections all appeared to be OK (green dots).
Now I just checked and the same is happening on my iPhone, I can receive but I cannot send emails. I suspect it may have something to do with iCloud Keychain, which I have enabled but I'm not sure and I haven't tried disabling it so far.
Anyone has had a similar problem and was able to resolve? Any suggestions?