I hit this bug/feature too. After upgrading fetchmail to 6.3.26-3, some inboxes were silently ignored. The usual error emails from fetchmail were not sent. I only discovered this several weeks later :(
Debugging shows: $ apt-cache policy fetchmail fetchmail: Installed: 6.3.26-3 Candidate: 6.3.26-3 Version table: *** 6.3.26-3 500 500 http://ftp.de.debian.org/debian stretch/main amd64 Packages 500 http://ftp.de.debian.org/debian sid/main amd64 Packages 100 /var/lib/dpkg/status 6.3.26-1+b1 500 500 http://ftp.de.debian.org/debian jessie/main amd64 Packages $ fetchmail -c -v -v -v Old UID list from email-server: <empty> Old UID list from localhost: <empty> Scratch list of UIDs: <empty> fetchmail: --check mode enabled, not fetching mail fetchmail: 6.3.26 querying email-server (protocol IMAP) at Tue 10 Oct 2017 10:33:30 AM CEST: poll started Trying to connect to 10.0.0.1/993...connected. fetchmail: OpenSSL reported: error:1417118C:SSL routines:tls_process_server_hello:version too low fetchmail: SSL connection failed. fetchmail: socket error while fetching from user@email-server fetchmail: 6.3.26 querying email-server (protocol IMAP) at Tue 10 Oct 2017 10:33:30 AM CEST: poll completed Merged UID list from email-server: <empty> fetchmail: 6.3.26 querying localhost (protocol IMAP) at Tue 10 Oct 2017 10:33:30 AM CEST: poll started OpenSSL shows that the server is using TLSv1: $ openssl s_client -host 10.0.0.1 -port 993 <snip> CONNECTED(00000003) <snip> No client certificate CA names sent Server Temp Key: ECDH, P-256, 256 bits --- SSL handshake has read 1286 bytes and written 310 bytes Verification error: unable to verify the first certificate --- New, TLSv1.0, Cipher is ECDHE-RSA-AES256-SHA Server public key is 2048 bit Secure Renegotiation IS supported Compression: NONE Expansion: NONE No ALPN negotiated SSL-Session: Protocol : TLSv1 Cipher : ECDHE-RSA-AES256-SHA Session-ID: 6E1A0000980314FC8F72FAE53500BA7018F4FDE74E04C428AD573A1CF5259CA8 Session-ID-ctx: Master-Key: 9532468A10452BCCB3812E8219DA407B01FC5303D5FFB4D934D6D7A9B1BC929D4A846D233EAB779C01367D6511CB7C01 PSK identity: None PSK identity hint: None SRP username: None Start Time: 1507625022 Timeout : 7200 (sec) Verify return code: 21 (unable to verify the first certificate) Extended master secret: yes --- * OK The Microsoft Exchange IMAP4 service is ready. Downgrading to fetchmail to 6.3.26-1+b1 is a workaround, as is adding --sslproto TLS1 command line parameter. fetchmail should really fail more loudly when there are ssl problems. -Mikko