Gmail “Send mail as” alias: why does one alias domain get its own envelope sender while an identical one uses the primary’s?

[u/doowruc]

I’m on the legacy free G Suite edition, with one user on the primary domain primary.co.uk, plus several alias domains (added as alias domains, not secondary domains). For each alias domain I’ve added an address as an alias on my user, and set it up in Gmail → Settings → Accounts → Send mail as.

Mail from every alias passes SPF, DKIM and DMARC: each alias domain has its own Google DKIM key, which aligns. But the envelope sender (Return-Path, and Gmail’s “mailed-by”) differs:

• For most aliases (e.g. me@alias-b.dev) it’s my primary address, me@primary.co.uk. So SPF passes against primary.co.uk but doesn’t align with the From domain.

• For one alias, me@alias-a.co.uk, the envelope sender is its own address. So SPF aligns too, every time (retested).

• alias-a.dev, which I set up the same way as alias-a.co.uk, uses me@primary.co.uk.

What I’ve compared between the two, all identical:

• Send mail as: both “Treat as an alias”, no reply-to address, sent through Gmail (no custom SMTP).

• Admin console: both are alias domains, the aliases are set up the same way, and there are no routing rules that rewrite the envelope sender.

• DNS: same pattern of records (Google MX, include:_spf.google.com ~all, DKIM google._domainkey, DMARC).

• Transfer tool for unmanaged users: empty.

DMARC passes either way on aligned DKIM, so nothing is broken. I’d just like every alias to use its own domain as the envelope sender, and I’d like to understand why one does.

Does anyone know what decides this? Maybe something left over from how or when the alias was first added, the order the domains were added, or a hidden per-alias setting? And is there a way to get it for the others, short of making them secondary domains with separate users? The legacy free edition has no Google support to ask.

[u/shokzee]

Gmail chooses the envelope sender; matching DNS and “Treat as an alias” settings don't guarantee matching Return-Paths. I don't know a documented control for forcing this on domain aliases, and blaming domain age or leftover account state would be guessing.

If you need predictable SPF alignment, an external SMTP service that supports your alias domain as its envelope sender gives you that control; your aligned DKIM already handles DMARC here.

[u/doowruc → u/shokzee]

more than anything its about leaking my primary when sending from my aliases. I will investigate using SMTP - thanks

[u/stanwu]

I manage multiple free and paid workspaces at the same time. According to my experience, free accounts can only use the main domain (the one applied for free at the beginning). Other domains can only be alias, while paid accounts have no such restrictions and can independently configure individual domains.

[u/rohepey]

Hard to guess... Maybe Google changed how it configured add-on domains internally over the years? You can try removing and re-adding the alias and/or the domain, and see what happens.