The Service desk sometimes doesn't accept email

Summary

Sometimes the Servis Desk rejectd the email. It behaves completely randomly. Sometimes the email is received, other times the email sent to the same address is rejected.

Steps to reproduce

Send email on any Servis Desk address - for example: gitlab+drupal-blog-aira-cz-108-issue-@aira.cz

What is the current bug behavior?

The email is rejceted, the boucne message is generated. It the gitlab-rails/application_json.log log is the message:

{"severity":"WARN","time":"2020-10-13T10:16:57.765Z","correlation_id":"077578b99148f1699f5494935cb0a8ce","message":"Email can not be processed: Gitlab::Email::ProjectNotFound

#### Results of GitLab application Check
System information
System:         Debian 9.13
Current User:   git
Using RVM:      no
Ruby Version:   2.6.6p146
Gem Version:    2.7.10
~~~
Bundler Version:1.17.3
Rake Version:   12.3.3
Redis Version:  5.0.9
Git Version:    2.28.0
Sidekiq Version:5.2.9
Go Version:     unknown

GitLab information
Version:        13.4.3
Revision:       6689552c479
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     11.9
URL:            https://gitlab.aira.cz
HTTP Clone URL: https://gitlab.aira.cz/some-group/some-project.git
SSH Clone URL:  git@gitlab.aira.cz:some-group/some-project.git
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers:

GitLab Shell
Version:        13.7.0
Repository storage paths:
- default:      /var/opt/gitlab/git-data/repositories
GitLab Shell path:              /opt/gitlab/embedded/service/gitlab-shell
Git:            /opt/gitlab/embedded/bin/git