PHP imap_open login delay (Cyrus IMAP)

Hello, I would like to ask you a question about the PHP imap_open call. I have an issue with imap_open producing an unreasonable delay when logging into the Cyrus IMAP server. The delay is exactly 5 seconds and it is constant across different connections. 1 0.000000 TCP 40496 > imap [SYN] Seq=0 Win=5840 Len=0 MSS=1460 TSV=5166 TSER=0 WS=6 2 0.588600 TCP imap > 40496 [SYN, ACK] Seq=0 Ack=1 Win=5792 Len=0 MSS=1350 TSV=3543919597 TSER=5166 WS=6 3 0.588659 TCP 40496 > imap [ACK] Seq=1 Ack=1 Win=5888 Len=0 TSV=5225 TSER=3543919597 4 0.597507 IMAP Response: * OK [CAPABILITY IMAP4 IMAP4rev1 LITERAL+ ID STARTTLS] Cyrus IMAP v2.3.14 server ready 5 0.597527 TCP 40496 > imap [ACK] Seq=1 Ack=127 Win=5888 Len=0 TSV=5226 TSER=3543919613 6 5.643035 IMAP Request: 00000000 LOGIN username password ^^^^^^^^ 7 5.750195 TCP imap > 40496 [ACK] Seq=127 Ack=35 Win=5824 Len=0 TSV=3543924696 TSER=5731 8 5.945506 IMAP Response: 00000000 OK ... The Cyrus IMAP server is working fine and responds without delay to non-PHP logins. On the other hand, when imap_open is called for Courier IMAP server, there is no 5 seconds delay and PHP logins proceed smoothly. The code I am using for PHP imap_open is <?php $host = "{10.10.10.10:143/notls}"; imap_open($host.mb_convert_encoding("INBOX", 'UTF7-IMAP','UTF-8'),"username","password"); ?> I am new to PHP and would be much grateful to receive pointers for debugging this issue. Thank you! Evgeniy