I researched this issue for some time because the errors initially started in Mojave and have continued after the update to Catalina. It popped up with an option to use a different smtp server a similar account is using, but that only worked part of the time. Using IMAP to set up the accounts is still the best option, but you need to ensure the SMTP is set up correctly.
1) Open Connection doctor in mail and run “Check Again”. This will show any accounts generating sending errors.
2) Once you’ve identified the problem accounts in Connection Doctor, double click on the account having issues and it will pull up system preferences for Mail.
3) Under the Accounts list in Mail preferences select the mail account in question and then select the “Server Settings” tab.
4) Under this list it will show the IMAP settings to receive mail. Make sure these credentials are set correctly.
5) Then below the IMAP settings to login and receive mail you will see a tab that says “Outgoing Mail Server (SMTP)”. Here you will need to ensure that the account is using the correct SMTP settings and you are not trying to use another accounts SMTP credentials to send mail.
6) Most of the time IMAP will automatically populate the data for the accounts SMTP settings. Make sure that the SMTP settings are using the correct login ID and password. Also make sure the “Host Name:" listed starts with SMTP and lists your mail account domain in the address. Once you’e verified all of the account login information and your SMTP settings are correctly make sure to check the box that says “Automatically manage connection settings” so you won’t have to enter your password each time you try to send mail from that account.
Hope this helps.
For example, Microsoft accounts use one specific server now for all of their pop accounts. For these accounts you will need to make sure the user credentials are set correctly and the Host Name address is “smtp-mail.outlook.com”.
