Simple Mail Transfer Protocol (SMTP) is a standard protocol for sending emails across the internet. It allows a sender to transfer emails to a recipient’s email server.
When sending emails via SMTP, there are some limits you need to be aware of:
Email Size Limits
One of the most common limits with SMTP is the maximum email size. Here are some key size restrictions:
- SMTP Server Limit – Each email server will have a maximum size for emails it can receive. This is often between 10-50MB. Going over this limit will cause the email to be rejected.
- ISP Limit – Internet Service Providers (ISPs) may also enforce limits, typically between 5-10MB. They will reject or throttle emails over the limit.
- Corporate Limit – Organizations often implement their own limits too, commonly around 25MB. This helps protect their email infrastructure.
So when sending emails via SMTP, you need to check the limits of the recipient’s email server, ISP and organization. Stay under the smallest limit to ensure deliverability.
In addition to overall email size, some SMTP servers also enforce limits on the number or size of attachments.
For example, Gmail only allows up to 25MB of attachments per email. Other providers may restrict emails to 5 attachments or less.
To avoid issues, limit attachments in size and number based on the recipient’s email system. If needed, split across multiple emails.
Email Recipient Limits
SMTP servers also commonly restrict the number of recipients you can send an email to at once:
- To field – Most SMTP servers limit the number of direct recipients in the To field to 50 or 100.
- CC field – The max for CC copies is often around 50-100 too.
- BCC field – For blind copies in BCC, limits can be 100-1000 depending on the server.
So if you need to send an email blast to a large list, you’ll need to segment it across multiple emails to avoid crossing these recipient limits.
Managing Large Email Lists
When you need to email a large subscriber list, use an email service provider (ESP) instead of SMTP. Options like MailChimp allow you to easily send to thousands of addresses while staying within major ISP limits.
Segment your list and send in batches of a few hundred. Avoid blasting thousands of recipients in one email via SMTP as this is often flagged as spam.
Sending Frequency and Volume Limits
To prevent spammers and abuse, SMTP servers implement various limits on how frequently you can send:
- Per Message – Most providers enforce a 5-10 messages per minute limit.
- Per Domain – There is also a cap on the total volume per sender domain, like 1000 per hour.
- Per IP Address – Shared IPs often allow around 150-200 per hour. Dedicated IPs can send more.
If you exceed the sending frequency or volume limits, your emails may be throttled or blocked entirely. Monitor your sending patterns and stay within restrictions.
Building Reputation Over Time
As you establish a good sending reputation over weeks and months, providers may incrementally increase your SMTP limits. But start slowly and focus on deliverability before trying to blast out huge volumes.
SMTP servers also implement various security limits:
- Authentication – Most require sender authentication via SMTP AUTH or other methods before allowing mail sending.
- TLS encryption – Many providers now enforce TLS encryption for increased security.
- IP reputation – Unknown or poor reputation IPs may face additional restrictions.
- Content filters – Servers often scan for spammy content, blocking emails that trigger filters.
Following security best practices like using TLS, authenticating properly, and avoiding spam content will help maximize deliverability when working within SMTP limits.
Managing Deliverability Within Limits
Here are some tips for managing SMTP limits and deliverability:
- Know the specific limits for each recipient domain and server.
- Segment large recipient lists across multiple emails.
- Limit attachment sizes and number.
- Pace sending at volumes within restrictions for your domain and IP.
- Enable TLS encryption and SMTP authentication.
- Build sending reputation slowly over time.
- Use an ESP for large email blasts.
- Avoid spam filters by sending valuable content.
While SMTP is a great protocol for transactional and promotional emails, senders must work within size, frequency, and security limits. The specific restrictions depend on the recipient’s email system and your sending reputation. With a deliverability strategy tailored to staying within SMTP limits, you can reach inboxes reliably.
- Email size, attachments, and recipient number per message are common SMTP limits to watch.
- Frequency and volume caps prevent abuse like spamming.
- Security measures like TLS and authentication enable higher limits.
- Build reputation slowly and use ESPs when emailing large lists.
- Know the limits for each recipient domain and craft deliverability approach accordingly.
Frequently Asked Questions
- What is the size limit for SMTP emails?
The maximum size for SMTP emails can vary between 10-50MB depending on the recipient’s email server and ISP limits. Stay under the smallest limit to ensure deliverability.
- How many recipients can you have in To/CC/BCC fields?
Most SMTP servers restrict the To field to 50-100 recipients, the CC field to 50-100 recipients, and the BCC field from 100 up to 1000 recipients depending on the server.
- What are some techniques to stay under SMTP limits?
Strategies like segmenting large lists across multiple emails, limiting attachment size/number, pacing sending volume, and using an ESP help observe SMTP limits.
- Does Gmail have a size limit for attachments?
Yes, Gmail restricts attachments to 25MB per email. Other common attachment limits are 5 attachments per message or 10MB per attachment.
- Can I get my SMTP limits increased over time?
Yes, as you build reputation, providers may incrementally increase limits. But focus on deliverability within current restrictions first before requesting higher limits.
- What causes the “Daily sending quota exceeded” error?
This error occurs when your domain has hit its daily sending limit quota based on volume and reputation. Reduce your sending or pace it out more gradually.
- Why do I need to authenticate when sending SMTP emails?
SMTP authentication helps providers verify your identity and comply with security best practices. It may allow higher sending limits compared to unauthenticated mail.
- Does using TLS encryption allow for higher SMTP limits?
In some cases, yes. Using TLS and other security measures indicates you are a trustworthy sender, which may lead to increased SMTP allowances.
- What is the limit on emails per minute I can send?
Most SMTP providers limit accounts to 5-10 emails per minute. Sending above this threshold may lead to throttling or blocked messages.
- How can I send to a list larger than 100 recipients?
For large lists, segment your recipients across multiple emails or use an email service provider that can handle larger volumes while observing ISP limits.
- What is the downside of putting 1000 recipients in BCC?
Putting over 100 recipients in BCC may exceed limits and be flagged as spam. For large lists use an ESP or segment into multiple emails under 100 BCC recipients each.
- Can I reuse the same SMTP servers when emailing large lists?
You should rotate and distribute large volumes across multiple SMTP servers/IPs to avoid exceeding any single provider’s limits.
- What happens if an email exceeds the recipient’s size limit?
If an email is over the recipient server or ISP size limit, it will bounce back as undelivered. Check the limit and keep emails under the max size.
- How can I find out the SMTP limits for a domain?
You can check for published limits on a domain’s website. Or attempt sending emails of increasing size and frequency to detect when they start getting rejected.
- What is a best practice for staying under frequency limits?
Monitor your messages per minute and pace out large volumes over an extended period. Tools like throttling can automatically help you stay under frequency caps.
- Should I avoid attachments when emailing large lists?
Yes, limit or avoid attachments when doing large email blasts as each recipient’s server needs to accept the total email size with attachments.
- What is a drawback of exceeding recipient SMTP limits?
If you exceed limits on recipients per email, total recipients per hour, or messages per minute, your emails are more likely to get flagged or blocked as spam.
Understanding and working within SMTP limits is crucial for successful email deliverability. Take the time to identify each recipient domain’s specific size, frequency, and security restrictions. Then carefully craft a sending approach tailored to staying within the bounds for both individual emails and overall sending volume. With smart limit management, you can maximize inbox placement while avoiding issues like throttling, blocking or spam filtering.