Outgoing email doesn’t get sent in Wi-Fi

This is the strangest thing! I cannot send out Comcast emails on my home Wi-Fi (Verizon FIOS). They are just sitting in the Outbox. As soon as I switch off Wi-Fi to the AT&T cellular network, the email gets sent out. I do receive them on Wi-Fi, but they are not being sent out. Another strange thing is it doesn't happen in someone else’s house when I am on their Wi-Fi. What can be wrong? I appreciate any advice anyone can give me. Thank you in advance!