This question already has an answer here: Dovecot pop3: Disconnected for inactivity 2 answers -I found server logged into mailbox on my smtp server -This server released connection for inactivity after 10 minutes. -in the 10 minutes between logged in & disconnected for inactivity, 3 attempts to send message from 3 different clients to this mailbox failed due to unable to lock for exclusive access: Resource temporarily unavailable -after disconnection the 3 messages reached mailbox good. I tried to simulate the process and lock test mailbox but I couldn't, I was aiming to understand who can lock ? who has exclusive access ? and why only client server can lock ? and how to solve this ?