SMTP Error 550 5.5.3 – Too Many Recipients: How to Fix & Avoid It

What is SMTP Error 550 5.5.3 – Too Many Recipients?

SMTP (Simple Mail Transfer Protocol) Error 550 5.5.3 means your email provider rejected your email because there were too many recipients. Think of it as an automatic safety mechanism (email service providers put these restrictions in place to reduce spam, prevent server overload and keep email security practices in check).

These limits, while maddening, are designed to protect the email system from abuse, both for the sender and for the recipient. These limitations are key to understanding how to properly execute bulk emails and outreach.

Let’s explore why this error happens and how different providers enforce sending limits.

Common causes of SMTP Error 550 5.5.3

SMTP Error 550 5.5.3 can be caused by various factors and are mainly related to email service provider (ESP) restrictions, security policies, and bad bulk email practices. The following are the most common reasons for this error.

Exceeding maximum recipient limits

The number of recipients for emails is strictly limited by email providers to prevent their users from receiving spam-like messages. Keeping a healthy email server performance is also another reason. 

For example, Gmail has an upper limit of 500 recipients per email for regular accounts (Microsoft 365 does too with a 500 recipient limit per email). When you exceed the limit, the attempt is automatically rejected, making it difficult for businesses and marketers to send mass mails in one go. 

Additionally, even business email providers such as Zoho Mail have daily sending limits and exceeding that can temporarily block the user account or suspend it as well. 

Bulk email sending without proper configuration

Authentication protocols, such as SPF, DKIM, and DMARC are used by email servers to validate a sender’s credibility. When emails in bulk are sent without these configurations (or incorrect ones), those emails often fail security checks and get marked as spam. 

This is especially true for newly created domains or mailboxes that haven’t built up a strong sending reputation yet. Also, even legitimate high-volume senders can see their emails being blocked or bounced if their configurations are incorrect.

Suspicious sending patterns

A surge of emails (for example, sending 10,000 emails all in one go from a new email account) can set off spam filters and security protocols. Email providers look for suspicious sending habits, such as sending the same messages to large lists of recipients, using deceptive subject lines or omitting unsubscribe links. One of the most common scenarios is an email marketer who goes from sending out 50 emails per day to 5,000 overnight.

How to fix SMTP Error 550 5.5.3

1. Reduce the number of recipients per email

• Split recipient lists into smaller groups (e.g., instead of sending to 500 at once, send to 100-200 per batch).
• Use email-scheduling tools to space out email sending over time.
• If using an ESP, configure settings to automatically segment your list.

2. Enable SMTP authentication

SMTP authentication (SPF, DKIM, and DMARC) helps to confirm that your emails are coming from a legitimate sender, so that they are less likely to be marked as spam. Email providers have implemented rules to help them determine who can send email on behalf of a domain, and without proper authentication, your messages may be rejected or wind up as spam.

• First, add SPF (Sender Policy Framework) records that will identify the sources authorized to send email.
• Configure DKIM (DomainKeys Identified Mail) to sign outgoing emails for verification.
• Implement DMARC (Domain-based Message Authentication) policies to monitor authentication status and prevent spoofing.

3. Optimize email sending frequency

Gradually increasing sending volume—also known as email warming—helps establish a positive sender reputation.

• Start with a small number of emails per day and gradually increase volume over a few weeks.
• Use Warmy.io’s AI-powered warmup service to automate and optimize this process.
• Monitor engagement rates (opens, clicks, bounces) to adjust sending frequency accordingly.

4. Check and adjust sending limits

Some email providers allow users to request higher sending limits, especially for business or verified accounts. If you’re frequently hitting recipient limits, upgrading your account or requesting a higher quota may be an option.

• Check your email provider’s sending limits and account type (e.g., free vs. paid).
• Contact customer support to request an increase in daily sending limits.
• Use Google Workspace, Microsoft 365, or a dedicated email relay service to expand sending capabilities.

5. Use multiple mailboxes for bulk sending

You can also distribute emails across other account types or domains. This is especially helpful for mass outreach campaigns. 

• Have different email addresses or subdomains set up for different campaigns. (For example, different email address for products, and different email address for promos)
• Rotate sending between different SMTP servers or ESP accounts to prevent hitting rate limits.
• Use Warmy.io’s multi-mailbox management to track and optimize deliverability across multiple mailboxes.

6. Ensure compliance with anti-spam regulations

Following email compliance laws not only prevents legal issues but also improves deliverability by reducing spam complaints and unsubscribes. CAN-SPAM (USA), GDPR (Europe), and CASL (Canada) regulations force businesses to send only certain opt-in emails only and provide clear opt-out options where applicable.

• Always obtain consent before sending bulk emails (avoid purchased email lists).
• Include an unsubscribe link in every email that is clearly recognizable.
• Maintain a clean email list by removing invalid, inactive, or unengaged contacts.
• Use double opt-in methods to ensure recipient engagement.

Best practices for bulk email sending

Preventing SMTP Error 550 5.5.3 isn’t just about not hitting sending or recipient limits—it’s about creating effective email deliverability to keep your emails going to inbox instead of spam. Here are best practices to follow when sending bulk emails:

1. Warm up new email accounts. Suddenly sending thousands of emails from a new mailbox can trigger spam filters and harm your sender reputation. Gradually increasing your email volume over days or weeks builds trust with email providers. Tip: Use Warmy.io’s AI-powered email warm-up to automate this process and improve inbox placement.
2. Segment your email list. Not all recipients are the same—one-size-fits-all emails can negatively impact engagement and increase spam complaints. Audience segmentation enables you to create more targeted, relevant messaging. Tip: Make sure to group your contacts based on industry, previous interactions, or engagement levels in order to support targeted emails.
3. Monitor bounce rates. High bounce rates indicate to email providers that you’re mailing addresses that are invalid or inactive, and it can negatively impact your sender reputation — potentially resulting in you getting blacklisted. Tip: Regularly clean your email list by removing hard bounces and non-responsive contacts. 
4. Personalize emails. Bulk emails that are not personalized are more likely to be flagged as spam. If you use the name of recipients, mention the name of their company or customize content in a way it caters to them, it contributes to greater engagement and inbox placement.

How Warmy.io helps avoid SMTP Error 550 5.5.3

When it comes to bulk email sending, deliverability is everything. You can have the best-crafted emails, but if they land in spam or get blocked due to sending limits, they won’t drive results. This is where Warmy.io comes in—an AI-powered email deliverability tool designed to warm up your email accounts, optimize sending limits, and ensure inbox placement.

AI-powered warmup process to optimize bulk email sending

Email providers closely monitor sending patterns, engagement rates, and authentication before deciding whether your email belongs in the inbox, spam, or should be rejected altogether. Warmy.io helps bulk senders avoid SMTP errors (like 550 5.5.3) by:

• Gradually increasing your email sending volume to build trust with ISPs.
• Mimicking human-like engagement (opens, replies, and marking as important) to boost sender reputation.
• Helping in configuring SPF, DKIM, and DMARC to prevent authentication issues.
• Providing real-time deliverability monitoring so you can optimize bulk sending strategies.