An integrated threat protection solution designed to detect, investigate, and respond to cyber threats across Microsoft 365 services.
Hello,
When you enter an email address in Outlook, a small circle with an x within usually displays to let you know if there is a problem with the domain or email address you are trying to send the message. It might imply that the domain is invalid or has been blocked, or that the email address you gave is inaccurate or does not exist.
In short, either blocked, or their domain provider is having issues, you mentioned this only happens on Mondays, they could be doing maintenance, but we can't say for sure.
Make sure their email and/or domain/website is not blocked in your email's blocklist, better yet add them to the trusted senders list.
Hope this helps,
JC