Thanks Mark. We're using an external host(s) for the outgoing MTA. See settings below. I can advise those sysadmins about your recommendations. We have whitelisted the Mailman host on the SMTP servers and I don't believe there are any connection limits or throttling being done on that end. Let me know if any of these settings seem wrong or problematic. Thanks again.
[mta] configuration: python:mailman.config.postfix [mta] delivery_retry_period: 2d [mta] incoming: mailman.mta.postfix.LMTP [mta] lmtp_host: 127.0.0.1 [mta] lmtp_port: 8024 [mta] max_autoresponses_per_day: 10 [mta] max_delivery_threads: 0 [mta] max_recipients: 500 [mta] max_sessions_per_connection: 0 [mta] outgoing: mailman.mta.deliver.deliver [mta] remove_dkim_headers: no [mta] smtp_host: smtp.caltech.edu [mta] smtp_pass: [mta] smtp_port: 25 [mta] smtp_secure_mode: smtp [mta] smtp_user: [mta] smtp_verify_cert: no [mta] smtp_verify_hostname: no [mta] verp_confirm_format: $address+$cookie [mta] verp_confirm_regexp: ^(.*<)?(?P<addr>[^+]+?)\+(?P<cookie>[^@]+)@.*$ [mta] verp_confirmations: yes [mta] verp_delimiter: + [mta] verp_delivery_interval: 1 [mta] verp_format: ${bounces}+${local}=${domain} [mta] verp_personalized_deliveries: yes [mta] verp_probe_format: $bounces+$token@$domain [mta] verp_probe_regexp: ^(?P<bounces>[^+]+?)\+(?P<token>[^@]+)@.*$ [mta] verp_probes: no [mta] verp_regexp: ^(?P<bounces>[^+]+?)\+(?P<local>[^=]+)=(?P<domain>[^@]+)@.*$