Hello,
With some POP3 servers, when you login more than once, the POP3 server says "Err: multiple login". How can I achieve that with dovecot ?
this is nothing that's really useful. At your ISP, this is probably due to a lockfile if messages are stored in the mbox-format, because if two processed would modify your mbox-file, the file would get corrupt. The disadvantage is clear: If the process dies for whatever reason, the lockfile wouldn't be removed and you could never login again until you call the pop3-administrator to manually remove the lockfile. Also, you couldn't check for new messages with webmail, while your email-client is downloading an email with a very large attachment. So far about the benefits and drawbacks of blocking multiple logins, however, I can't tell you if dovecot is capable of preventing parallel logins.
Regards Marten