Emails not being received by a Third party

Good morning,

My company is using Microsoft B2C as a login for our web app.  We have found that there are a couple email providers that are having an issue with receiving our Password reset and Account activation codes.  Most do not receive a password reset and most receive the Account activation codes but its 5 hours later and the code is expired.  My question is, have you seen any issues or have any information around this that may help?