Not receiving emails on Gmail?

Question

gmail

Welcome all. SUCH question, created a unique mail for his site on the gmail platform. Assigned an address to the mail instead of gmail.com, assigned a domain name. It looks like this " [email protected] ".
Noticed the problem after a few weeks. Some letters arrive at the post office, some do not reach at all or arrive with a delay of several hours. I tied up a unique name for mail through the mail client " Mozilla Thunderbird "
How to fix it? What needs to be done and what is the problem?