Okay, you seem stuck with mail.
Every Mail app and every mail client tool has within it two separate groups of credentials and mail server settings. One group is used for reading and receiving mail. The other group for sending mail. The receiving group are for either IMAP or POP servers, and include the login username, password, host name of the server, the server port, and the connection security setting. The sending group is for SMTP or ESMTP servers. There’s a username, password, server host name, port, and connection security setting over there, too.
Yes, some folks use different users and passwords for sending and receiving mail, as bizarre as that might seem.
There’s no single way to set up a mail server, which means there are routinely differences specific to the selection of mail servers you’re using.
Your password here is (usually) set by your mail provider, though some providers allow you to set the password using a web interface. This reset might be via a web mail interface, or via a web “portal” site such as that offered by Yahoo. Or by asking the mail provider support folks for help.
All of that is a long way to stating the following... One of your passwords or maybe the username is incorrect, or some other setting has changed. Or maybe the server—the IMAP server, in this case—is having issues.
Here’s the Apple documentation on this, and it’s your read/receive credentials or password that are incorrect:
If you can’t send or receive emails on your iPhone, iPad, or iPod touch - Apple Support
Or the shorter version, from the error message... Fix your IMAP password. Or check with your mail provider.