Invalid/Non-functioning E-mail error

We had an enquiry through our contact form but the person did not check the 'Allow further email' checkbox.

She now wants to be advised of future meetings so I am trying to add her to the Prospects list but I am getting this error:
You have errors in your form below. Please scroll down and correct the following:

Invalid/Non-functioning E-mail: <username>@memyhealthandi.org (No advertised SMTP servers are accepting mail.)

I know the email address works as I have corresponded with her and copy/pasted from her email.

I see from previous posts that this can sometimes be caused when a mail server is blocked.

Is that the case here?

I will need the full email address if I am to find the issue.

The username part is emilie.marie

1) her mail server is VERY slow
2) she is hosting at a datacenter that spams FTH frequently and the mail server IPs were blocked.
I have removed the block for now but it can be blocked again at anytime.