Many people like to use forwarders so that they only have 1 email account to check. Sometimes problems occur with this.
Unfortunately, forwarding to gmail accounts is problematical. Forwarding to yahoo.com is even worse. The 2 things which commonly go wrong is gmail deciding forwarded mail is spam and deleting it and the other throttling sending rates.
The cause of the problem is people complaining in their gmail account about spam which has been forwarded to them. We do fairly well at stopping spam, easily 98% and that is as good as anyone, including gmail. But when several or many people mark the remaining spam as what it is, gmail regards the likelihood of mail coming from our servers as being spam much higher. That is, they think spam just came from our server, the probability that the next incoming emails are also spam is much higher. That’s incorrect in this situation, but you can see why it might seem to make sense. When their system reaches a probability high enough, they start throttling and even discarding emails. They would discard emails when other characteristics of a particular email also suggest that it’s spam.
I have sent emails to them several times asking what might be done about the problem. The only response I have gotten is being referred to their bulk mail guidelines. Not applicable and quite pointless. This is not an unusual problem. They ought to account for it, but they don’t.
There are a couple of things you can do. One is to stop using gmail directly by setting up a forwarder:
You would then check the account which you set up as the forwarding target instead of gmail.
The remaining things you can do are to use an email client instead of web mail and set it up so that it checks multiple accounts or simply check several places.