Deleting old accounts from Mail I have two accounts in Mail (v. 10.3), on a MacBook Air 13" running macOS 10.12.6. One is a work-related account with a POP configuration and the second one is from a hobby-group with an IMAP configuration. The work account has changed servers several times in the last few years. When the work account changed servers, I would add the new account, and leave the old inactive accounts on the mail app. 

Today, I tried unsuccessfully to modify an existing account with the new servers addresses. I looked up analogous situations here and on other Mac forums to find out how to deal with my old accounts. I am worried that if I remove them completely, I will lose old emails. 

Can I continue adding accounts with very similar information to old accounts without running into trouble? If not, how can I keep all my old messages under the most recent work account?
I would remove the old accounts for the sake of organization, especially if their details, or even addresses are the same (it sounds like they might be, just with different mail server configurations). You can archive old emails on your mac using the mail app. With the mail app open, go to the menu, click Mailbox -> New Mailbox. This will allow you to create a local mailbox:



Click OK, and the mailbox will be created. Copy all emails you want into this mailbox. It may help to create multiple local mailbox for organization purposes. Once all of your old emails are moved to local storage, you can remove the old accounts while keeping all of your old emails safely on your Mac.