Only NF emails not sending, other types work fine

Hello. We recently started having trouble with Ninja forms refusing to send emails. All other emails on the site work fine, and I am able to log in and send an email from that mailbox using webmail.

Ninja Forms is v. 3.6.14
PHP is v. 7.4.30
WP Mail SMTP is v. 3.5.2

We recently installed the WP Mail SMTP plugin to see if that would help, and it didn’t. The test emails work fine, lost password emails work fine. Here is the log from that plugin:

Mailer: Other SMTP
SMTP Error: data not accepted.SMTP server error: DATA END command failed Detail: Request failed; Mailbox unavailable
SMTP code: 550
Debug Output:
Connection: opening to ssl://smtp.bizmail.yahoo.com:465, timeout=300, options=array()
Connection: opened
SERVER -> CLIENT: 220 smtp.mail.yahoo.com ESMTP ready
CLIENT -> SERVER: EHLO leadingedgeresumes.com
SERVER -> CLIENT: 250-hermes--production-ne1-544744cc75-n6tzz Hello leadingedgeresumes.com [ip address redacted])
250-PIPELINING
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-SIZE 41697280
250 AUTH PLAIN LOGIN XOAUTH2 OAUTHBEARER
CLIENT -> SERVER: AUTH LOGIN
SERVER -> CLIENT: 334 VXNlcm5hbWU6
CLIENT -> SERVER: [credentials hidden]
SERVER -> CLIENT: 334 UGFzc3dvcmQ6
CLIENT -> SERVER: [credentials hidden]
SERVER -> CLIENT: 235 2.7.0 Authentication successful
CLIENT -> SERVER: MAIL FROM:
SERVER -> CLIENT: 250 2.1.0 Sender OK
CLIENT -> SERVER: RCPT TO:
SERVER -> CLIENT: 250 2.1.5 Recipient OK
CLIENT -> SERVER: RCPT TO:
SERVER -> CLIENT: 250 2.1.5 Recipient OK
CLIENT -> SERVER: DATA
SERVER -> CLIENT: 354 Ok Send data ending with .
CLIENT -> SERVER: Date: Fri, 16 Sep 2022 14:15:26 +0000
CLIENT -> SERVER: To: recipient email address redacted
CLIENT -> SERVER: From: Leading Edge Resumes
CLIENT -> SERVER: Reply-To: "reply address redacted"
CLIENT -> SERVER: Subject: Contact from LeadingEdgeResumes.com
CLIENT -> SERVER: Message-ID:
CLIENT -> SERVER: X-Mailer: WPMailSMTP/Mailer/smtp 3.5.2
CLIENT -> SERVER: X-Ninja-Forms: ninja-forms
CLIENT -> SERVER: MIME-Version: 1.0
CLIENT -> SERVER: Content-Type: text/html; charset=UTF-8
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER: Name:
CLIENT -> SERVER: Emily Birt
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER: Email:
CLIENT -> SERVER: email address redacted
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER: Telephone Number:
CLIENT -> SERVER: redacted
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER: Time Zone:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER: Best Time to Call:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER: City/State/Country:
CLIENT -> SERVER: redacted
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER: Target Position:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER: Highest Degree Earned:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER: Current Situation:
CLIENT -> SERVER: blah. ugh
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER:
CLIENT -> SERVER: .
SERVER -> CLIENT: 550 Request failed; Mailbox unavailable
SMTP ERROR: DATA END command failed: 550 Request failed; Mailbox unavailable
SMTP Error: data not accepted.

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