An email signature should change with the email it's on

Email signature best practice in 2026 comes down to whether this person has ever heard from you before. If they haven't, the signature should be almost nothing: a name and a company, in plain text. If they have, it can carry more, as long as it stays text and survives dark mode. The advice online splits into two camps because each one is describing a different email and calling it the whole subject.

The split runs through the research itself. A branding agency founder says a bad signature on mail to a client or a lead means you will not get a reply. A sole trader starting a hearing clinic spent longer than they wanted on credentials, logo and job title, because the signature "lands in the inbox of every care home manager, GP, and potential client you contact. It's working when you're not." In the same months, cold email operators were cutting signatures down to a name and a company. Both are right about their own email.

For customer mail, the standing line most service owners add is a review link. Treat that line as a reminder. The reviews themselves come from asking. A parking lot striping contractor got a review from all five businesses on one job by asking in person and texting the link while standing there, and their view of the alternative was blunt: "nobody is leaving a review 3 days later off an email." We looked at the review systems that do work in local business marketing.

The strongest evidence we found is about cold email that went out with a full signature. None of it comes from a controlled test. Each case is one sender's or one auditor's own account.

• The newsletter look. An agency operator describes a client sending HTML emails with logos and buttons that looked like a newsletter. Stripped to plain text, one link and no signature, the reply rate went from 0.8% to 3.1% in 10 days, by that operator's account.
• The block at the bottom. A founder whose team audits cold email setups sent a bland test note from the client's own account to a Google and a Microsoft inbox, and it landed. The real campaign did not. Removing one part of the email at a time, they found the signature: "the second we stripped the signature out, everything dropped into primary." A mailbox provider reports tracing another campaign's spam problem to the signature line alone.
• The phone number. An outbound agency discovered that a client's phone number in the signature had been flagged by carriers, and in their telling that one number pulled the whole sending domain into spam.
• The booking link. A mailbox provider claims raw links in a first cold email, a booking link included, burn the sending accounts within 6 to 8 weeks, and tells senders to ask for a reply instead.

In the audits we researched, the signature was the part nobody suspected: the setup was clean, the copy read fine, and one block of text at the bottom was sending the campaign to spam. Two of those accounts come from companies that sell sending infrastructure, so they have a reason to find problems. Still, every account points the same way, and we didn't find one person arguing for a full signature on a first cold email. If we were starting a campaign tomorrow, it would go out with a name and a company and nothing else.

What operators keep is a way to be checked. Several put their LinkedIn handle at the bottom as plain text, with no link. A prospect who wants to check out a stranger will search for it, and nothing in the email looks like a tracked URL. One team sending from several mailboxes uses the company's handle when a sender has no profile of their own. The pitch that used to sit in a banner moves into a P.S., where it reads as part of the message. For the emails after the first one, see the follow-up email after no response.

You design a signature once. The recipient's mail app then redraws it in the recipient's theme, and every rendering failure we found happened at that step.

• Coloured text in dark mode. Mail apps flip default text for a dark theme but leave text that has a colour set on it. One reader had to ask a sender for their name and business. Both were right there in the signature, in dark grey, invisible on a dark background.
• Outlook's two themes. One Outlook user found that the signature and the message do not follow the same light or dark setting, and ended up keeping a separate signature for each.
• Embedded logos. Gmail deliberately blocks images embedded in the message as base64, which is why a logo turns into a broken box for some recipients, and some spam filters flag a message that contains one. A small software shop's fix is to host the image at a normal address, or set the wordmark as text. Text doesn't get blocked.

So the client signature that lasts is plain text in the reader's default colour. Something like this:

Jane Doe
Owner, Doe Plumbing
(555) 010-0199
doeplumbing.com

Titles are where people overreach. A sales manager tells reps not to add "MBA", and a lawyer admits they can't take colleagues who overuse "Esq." seriously. Licence and registration numbers are the exception: a Texas real estate broker describes signature disclosure as one of the compliance questions agents ask most, so where a regulator expects the number, it belongs there.

If you sell software to service companies, the most-read signature in your customers' business belongs to your product. It's the line added to every quote, invoice, reminder and reply the product sends for them, and their own clients read it.

That line can be distribution. A podcast host who interviewed the founder of Superhuman reported in June 2026 that the email app still gets about 20% of its site traffic from "Sent by Superhuman Mail." The founder's explanation, as the host tells it, was identity. The line became a signal that the sender cares about their tools. There's a cost, too. Recipients tell senders to take such lines out, and "Sent from my iPhone" has become shorthand for never having opened the settings. The founder of one email sending tool describes the usual bargain: the footer appears only on the free plan, which includes 3,000 emails a month, and paying removes it. A footer spreads when users would choose to show it. Otherwise it's a charge they pay to get rid of.

Mail written by an AI agent gives the footer a second job. In the EU, Article 50 of the AI Act has applied since 2 August 2026: people must be told they are dealing with an AI system from the first interaction, unless that is obvious. An agent answering a client's email is exactly that interaction, and the footer is where the telling usually goes. Plenty of senders aren't sure what to do with it. One founder of an outbound agent product changed their signature from one naming the agent to their own name, and wrote: "Not sure this is the right call. May go back." A freelancer whose assistant signed a client reply "Your AI Assistant" says the client "thought I outsourced my whole brain." Some recipients now put the opposite line in their own signatures, asking not to be sent AI-written mail at all.

Say an accounting firm asks us to set up an agent that answers routine client questions. The footer we'd put on those emails is one plain sentence: the agent's name, the fact that it is an AI, and the firm it works for, with the partner's email for anything the agent should not answer. No banner or link, and nothing promotional inside the disclosure. Whoever reads it is deciding whether they still trust the firm. A disclosure that doubles as an advert makes that harder.