Mail sends ok but does not arrive - just to one address!

I can see from your post that you're unable to send emails to one recipient although the settings are correct and you're able to send to others with no issue. Being able to send emails can be crucial for communication and you've come to the right place for support. To get a better understanding of what's going on, please reply to the following question.
- Do emails send if you reply to a received message instead of composing a new one?

Additionally, here are some resources that can help:
If you can’t send or receive email on your Mac
Get help using iCloud Mail
Specifically, use these steps:

If your recipient doesn't receive your message

1. If an “unknown address” or “undelivered mail returned” message appears in your Inbox after you send a message, go to your Sent mailbox and make sure that the recipient's email address is correct.
2. If you're using OS X Mail or accessing your iCloud Mail using Safari or another supported web browser, check your Drafts, Sent, and Outbox mailboxes:
   • If you see an unsent copy of the message in Drafts, try to send it again.
   • If you see the message in your Outbox, make sure that you're connected to the Internet and use the steps above to select the iCloud SMTP server.
3. Ask the recipient to check their Junk mail folders or mailboxes.
4. If your recipient still doesn't receive your message, but your message was sent without any alerts or “undelivered mail” messages, your message might have been blocked or filtered by the iCloud Mail servers or the recipient's mail server. To find out, contact iCloud Support. You should also ask your recipient to contact their email administrator.

Let us know the results of trying these steps.
Take care!

I have the same problem but in reverse- mail from one particular IMAP account will receive but not send.

Message in Mail>Preferences>Accounts>Servers settings is "Unable to verify account name or password." Identical credentials work fine on iPhone.