My outgoing emails are randomly being blocked by Gmail

This is frustrating, and seems very random.

I use the Mail app, and Comcast is my ISP and email server.

I'll be having a conversation with someone. Back and forth.

But then at some random point,

I hit replay and send back an email, and then they never get it.

Or even if it's a new message.

Sometimes there are links in the content, sometimes not-

it seems to be more common with Gmail recipients, but has also happened with Yahoo recipient, during a conversation.

No links, just text.

I use an email signature, and have never had a problem, but wonder if there is an issue now?

They show as having been sent in my mail files.