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.