Sending mail leads to timeout

[k1ba-meow]

On a freshly installed dokuwiki (2020-07-29 "Hogfather", installed from Arch packages), after installing the smtp plugin, every action that would result in a mail being sent instead causes the nginx server to generate a 504 timeout response. Since this also prevents me from seeing the debug logs that would be rendered to the resulting page I'd be grateful for pointers where I can find more debug logs. Nginx error logs produces this, which seems fairly unhelpful:

2022/03/29 18:15:09 [warn] 759523#759523: *132077 an upstream response is buffered to a temporary file /var/lib/nginx/fastcgi/5/04/0000000045 while reading upstream, client: [IP], server: [URL], request: "GET /lib/exe/css.php?t=dokuwiki&tseed=565e66c98e1023f346e7ffba92c76e05 HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm7/php-fpm.sock:", host: "[URL]", referrer: "https://[URL]/"
2022/03/29 18:16:01 [error] 759523#759523: *130279 upstream timed out (110: Connection timed out) while reading response header from upstream, client: [IP], server: [URL], request: "POST /doku.php?id=start&do=admin&page=smtp HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm7/php-fpm.sock", host: "[URL]", referrer: "https://[URL]/doku.php?id=start&do=admin&page=smtp"