When receiving a large number of messages, some are missed until client is closed/opened.

[djcastaldo]

When there are a number of new imap messages at the same time (10+), Thunderbird doesn't immediately retrieve them all, either in the inbox, nor subscribed folders. It usually gets most of them but a few are missing until disconnecting/reconnecting or close/open Thunderbird and Get Messages again. It also works to restart the stalwart-mail server.

A workaround for this is to disable the Thunderbird setting for "Allow immediate server notifications when new messages arrive" and set the polling interval to 1 minute. With this setting, all the new mail seems to come in... (but, obviously, there is a slight delay). I am asking here if there is a stalwart setting I can use to make Thunderbird work better with the "Allow immediate.." setting enabled.

[mdecimus]

You should ask on the Thunderbird mailing list. Stalwart has no control of how clients retrieve messages. It also imposes no restrictions on the amount of messages that can be downloaded at once.