Wordpress Mail Bounces, SMTP Errors

My company is using Wordpress v5.4.1, Ninja Forms v3.4.24.3, Easy WP SMTP v1.3.9.3 and WP Mail Logging v1.9.6. We are having continuing intermittent problems with SMTP email bounces. The error is "SMTP Error: Data not accepted". In more detail: SMTP ERROR: DATA END command failed: SMTP Error: data not [login to view URL] -> SERVER: MAIL FROM: CLIENT -> SERVER: RCPT TO: CLIENT -> SERVER: DATA SMTP ERROR: DATA command failed: 250 Accepted SMTP Error: data not [login to view URL] -> SERVER: QUIT SMTP ERROR: QUIT command failed: 354 Enter message, ending with "." on a line by itself Preferably you have access to another mail server that accepts SMTP and we can temporarily route emails to your server. That would help us to isolate if this problem is coming from our Wordpress forms or is something misconfigured in the mail server. During the contract period I am prepared to share Wordpress credentials. I can consider sharing CPanel credentials but will need to change my password as they only allow a single account. I propose to set a payment milestone of 24 hours without email fails on Wordpress. I need to resolve this asap, so will give preference if you can start right away.