A Windows Mailer That Sends One Personalised Email at a Time from Your Own Mailbox

The client had a spreadsheet of contacts and needed each of them to receive a personal email from their own business mailbox — not a marketing blast from a third-party platform with someone else's sending domain on it. One recipient in To, nobody in CC or BCC, a copy in Sent Items, exactly as if it had been typed by hand.

What we built

A Windows desktop application with a CLI sharing the same engine, so both behave identically. It sends through the user's own Microsoft 365 mailbox over SMTP, files each copy to Sent via IMAP, and later re-reads that same mailbox to pull bounced addresses back out into a delivery summary.

• List handling — CSV or Excel input, reporting how many addresses were valid, how many rows were skipped and why, and how many recipients will be greeted by name. Optional deduplication to one email per company.
• Personalisation — a {first_name} placeholder, an attachment, and an embedded logo whose image travels inside the message rather than pointing at a file on the sender's own PC (the classic reason a logo pasted from Word arrives broken for everyone else).
• Typography controls — paragraph gap and line spacing, because a formatted email that reads well on a desktop can read cramped on a phone.
• Safety controls — a preview mode that renders finished messages without sending, a send-test-to-me button, a per-run cap, a configurable gap between sends, and a Stop button that takes effect immediately.
• Send-once guarantee — every successful send is recorded locally, so the same person is never emailed twice across runs.

Getting the greeting right

The greeting is derived from the email address itself, not from the name column, because in lists of this kind the contact name frequently sits beside an address belonging to somebody else. thomas.mugford@… becomes "Hi Thomas". Shared mailboxes like info@ or rental@ fall back to "Hi there". Ambiguous forms like jsmith@ also fall back, because an initial and a surname cannot be split reliably — and a missing name reads better than the wrong one. Single-word addresses are checked against a list of roughly 22,000 first names, which is how matthew@ is recognised while jsmith@ is not. Names found in the spreadsheet's own contact columns are trusted too, and the whole name list can be edited by the operator and re-loaded.

Deliverability, honestly

The documentation we shipped tells the user the uncomfortable truth rather than promising volume: Microsoft caps a normal mailbox at roughly ten thousand recipients a day, and a mailbox that usually sends a handful suddenly sending thousands is exactly what triggers throttling. The advice is to start at a few hundred a day, watch the bounce numbers, and raise the ceiling only if they stay low.

Testing and packaging

The project includes a fake SMTP server used to exercise the full send path without touching a real mailbox, and two review passes were run over the code specifically to close paths that could send a duplicate. It ships as a single PyInstaller executable, so there is no Python installation for the end user.

The application was built, packaged and reviewed. We hold no acceptance record for it, and the recipient data it was tested against is the client's — nothing from it appears here or in any screenshot.