Definition
An email alias is an alternative address that delivers messages to a primary mailbox without requiring a separate inbox. The most common form for consumers is subaddressing, also known as plus addressing — the practice of adding a tag after the plus sign in the local part of an email address, such as yourname+shopping@gmail.com. Gmail pioneered this feature, and it is supported by Outlook.com, Fastmail, and some other providers, but notably not by Yahoo Mail or many corporate Exchange servers. Subaddressing allows users to create unlimited unique email addresses that all route to their primary inbox, making it a popular tool for organising email and tracking which services share or sell subscriber data.
Subaddressing and email aliases create several challenges for email marketers. When the same subscriber signs up for different services using different subaddresses — yourname+sale@domain.com and yourname+news@domain.com — the email marketer sees two distinct subscribers even though they are the same person. This leads to duplicate content being sent to the same mailbox and inaccurate subscriber count reporting. Additionally, subaddressing can break email deduplication logic, cause issues with identity resolution in customer data platforms, and complicate suppression list management. Some email marketers strip plus tags from email addresses to normalise them for deduplication, though this practice is controversial because it may unintentionally merge distinct identities in rare cases where the plus tag represents a genuinely separate user.
Best Practices
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Strip plus tags from email addresses during signup and list import for deduplication: Normalise all email addresses by removing the subaddressing tag and any characters between the plus sign and the @ symbol. This prevents the same person from appearing multiple times in your list under different subaddresses. Apply this normalisation during signup, list import, and before sending to ensure consistent subscriber identity.
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Check whether your ESP or CRM supports subaddressing natively: Some modern email platforms and customer data platforms recognise subaddressing and can automatically group subaddresses under a single subscriber profile. If your platform supports this, enable the feature rather than implementing manual stripping, as it preserves the original address for transactional sends while unifying the subscriber view for marketing.
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Consider the privacy implications before stripping plus tags: Some subscribers deliberately use subaddressing to track which brands share their data. Stripping the plus tag removes this tracking capability and may violate user expectations. If your privacy policy or brand relationship depends on transparency, inform subscribers that you normalise email addresses and explain why.
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Test whether your email validation service handles subaddressing correctly: Some email validation services reject valid subaddresses as invalid because the plus sign is technically a special character. Verify that your validation provider correctly accepts Gmail subaddresses and other legal plus-addressed emails before implementing validation at signup.
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Monitor for alias abuse in your signup forms: Because subaddressing allows unlimited addresses, the same person can sign up for multiple free trials or promotional offers using different subaddresses of the same primary address. Implement cross-referencing that detects when the same normalised email appears multiple times and apply your duplicate account policies accordingly.
Related Glossary Terms
BIMI
BIMI (Brand Indicators for Message Identification) is an email standard that allows brands to display their logo next to their emails in supported email clients. It requires DMARC enforcement at quarantine or reject policy.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.
Email Deliverability
Email deliverability is the ability to land in the recipient's inbox rather than the spam folder. It depends on sender reputation, authentication, list quality, and content practices.
Double Opt-In
Double opt-in (also called confirmed opt-in) is an email signup process that requires a new subscriber to confirm their email address by clicking a verification link in a confirmation email before they are added to your mailing list.
Email Authentication
Email authentication (SPF, DKIM, DMARC) is a set of technical protocols that verify an email is genuinely from the domain it claims to be sent from. It prevents spoofing, phishing, and improves deliverability.
Email Bayesian Filtering
A statistical spam filtering method that uses probability calculations based on word frequency analysis to classify email messages as spam or legitimate.
Frequently Asked Questions
Subaddressing, commonly called plus addressing, is a feature that allows users to add a tag after a plus sign in the local part of their email address. For example, jane+deals@gmail.com delivers to jane@gmail.com. This lets users create unique addresses for different purposes while receiving all email in a single inbox.
No. Gmail, Outlook.com, and Fastmail support plus addressing, but Yahoo Mail and many corporate Exchange servers do not. Some providers use alternative conventions — for example, Yahoo does not support the plus sign as a subaddressing delimiter. Additionally, not all email marketers handle subaddressing consistently, which can cause deliverability issues.
They create duplicate subscribers. When the same person signs up for different campaigns using uniquely tagged subaddresses, your ESP treats each subaddress as a separate subscriber. This inflates your list count, causes the subscriber to receive duplicate emails, and leads to inaccurate engagement reporting.
In most cases yes, to prevent duplicates. Normalise addresses by removing everything between the plus sign and the @ symbol. However, be careful if your audience includes users who rely on subaddressing for privacy tracking — they may not appreciate having the tag removed. Consider informing subscribers about your normalisation practice.
Yes. Subaddressing allows the same person to sign up for multiple free trials, promotional offers, or bonuses using different addresses that all route to the same inbox. Businesses that offer first-time discounts or trial extensions should monitor for this pattern and implement deduplication based on normalised email addresses.