There are several ways for an email client to connect to one's mailbox. These modes are called "protocols".
Some protocols are very old and do not support the protection provided by MFA. POP3/SMTP and IMAP are examples of such protcols and are tupically called "legacy".
Legacy modes are no logner allowed beacuse legacy-based applications bypass the protection provided by MFA. An attacker could still take over such an account and send out phishing emails impersonating the account owner which makes the phishing attack much harder to detect. Additionally, an attacker could potentially access sensitive information within the account.
In order to secure the mailbox, if your email client uses legacy mode to connect to your EUI email address, the connection will NOT be allowed!