Anyway

This took me a while to fig­ure out yes­ter­day, so in the hopes it helps someone else (or reminds me of the solu­tion when I need to solve the same prob­lem again).

The symp­tom: Mail.app on my Mac laptop is send­ing email with the wrong “From” head­er, even though I’ve chosen the right account to send it from, and the email in the Sent folder has the right email address in the From header.

The back­ground: my major cli­ent com­pany wants me to start using their email address for work I do for them, which seems com­pletely reas­on­able to me. For vari­ous reas­ons, how­ever, I need to keep using the Google SMTP serv­er that I have for my Tex­tu­al­ity email to send the mail (SSL cer­ti­fic­ate stuff that I have no con­trol over being the main reason). 

The solu­tion: as far as the Google SMTP serv­er is con­cerned, you need to prove you own the email address that’s in the From head­er, or it will simply sub­sti­tute the email address that it does know that you own (in my case my textuality.com email address). To authen­tic­ate, and thereby allow the Google SMTP serv­er to use the oth­er email address as the sender email address, log in to the browser inter­face of your Google email account (wheth­er GMail or a Google Apps account). Click on the “Set­tings” link (should be top right). Click on the “Accounts” link. In the “Send mail as:” sec­tion, click on the “Add anoth­er email address you own” link. The help inform­a­tion on this does­n’t make it clear that this sec­ond­ary email address can be any email address, served through any sys­tem. It does­n’t have to be a Google account, it just has to be an address you receive email at. You won’t be pick­ing up that email using the Google sys­tem unless you also set up some­thing in the “Get mail from oth­er accounts:” sec­tion, which is not some­thing I wanted to do. Then, fol­low the instruc­tions. Google will send email to your sec­ond­ary address as a simple authen­tic­a­tion (veri­fic­a­tion) pro­ced­ure. Click on the link in the email, or fol­low the oth­er instruc­tions. Then hey presto, the Google SMTP serv­er will allow you to send your email using your sec­ond­ary email address as the “From” sender. And people reply­ing to it will send the email to the right account.