check and repair dovecot mail server

On a CentOS Web Server with DirectAdmin Control Panel there are some issues with the mail server lately. More specifically, yesterday I realized that nobody could send or receive emails, but nothing wrong on the mail clients! After complaints I had a look on the Email queue administration through DirectAdmin control panel and I saw a huge list of emails there. I restarted the dovecot service and then I started "retries" with the huge list of queue emails. After several hours I cleared that and everything was working fine, until this morning! So I need a system administrator that has good knowledge of CentOS/DirectAdmin Web Servers in order to have a look and check my mail server/service. For sure there is something that causes the problem and I want to solve it. My Web server has dovecot, FetchMail mail retrieval, Procmail Mail Filter and Spamassassin mail filter. I need all of them. I do not want to change anything on the configuration, as the server was working fine for months (more than a year). I just need to find and solve the problem that causes the mail server stop sending/receiving emails. Thanks a lot in advance for your time and your bid.