🐛 SMTP keepalive causes 421 timeout on first send after idle; subsequent sends work (1.9.0 self-hosted)

**Note:** This bug report was filed using AI assistance. The bug itself was triaged and verified by a real human (encountered in production, reproduced, workarounds tested). ## 🐛 Describe the bug On self-hosted Appwrite **1.9.0**, the **first** mail send after a period of idle time (e.g. an OTP, magic-URL link, verification email) fails with an SMTP **421 timeout** error. Subsequent sends in the same window succeed. The pattern matches a stale-keepalive bug: `worker-mails` keeps the SMTP connection open between sends (PHPMailer `SMTPKeepAlive`), but either the upstream SMTP server or a NAT in between has dropped the idle connection. The worker still tries to reuse the stale socket on the next send, the `MAIL FROM` command hangs, and the server returns `421 — closing connection`. The first failed attempt closes the connection client-side; the next attempt opens a fresh connection and succeeds. End-users see "OTP not delivered" → retry → works. ## 👟 Reproduction steps 1. Self-host Appwrite 1.9.0 via the official Docker compose. 2. Configure an SMTP provider via the standard `_APP_SMTP_*` env vars (any provider — reproduced against ours - Resend). 3. Trigger a send (e.g. request an OTP, password recovery email, or invite). 4. Wait long enough for the connection to be considered idle by either the SMTP server or any intermediate firewall/NAT (typically several minutes; varies by provider). 5. Trigger another send. Result: the first send after the idle window fails with the `421` log line (see below). The next attempt immediately after succeeds. ## 🔬 What we've ruled out - **SMTP credentials**: subsequent sends through the same worker process and same provider succeed, so creds and TLS are correct. - **Provider rate-limiting**: 421 timeout messages come immediately on `MAIL FROM`, not after a quota threshold; provider dashboard shows no rate-limit events. - **Network reachability**: outbound SMTP from the host works (`openssl s_client -connect` to provider port succee…