Emails go through an intensive process to get from inbox to inbox. This means that there are many factors that can delay a message.
Message Size
The size of the email message will affect how long it takes to send or receive. If you have an email that has multiple images or files attached to it, you may want to consider reducing the number of items attached or reducing the size of the attached item.
The most popular email providers have an upper limit for incoming emails:
- Gmail: 25 MB.
- Outlook/Hotmail: 10 MB.
- Mail.com: 50 MB. However, each file can only be 10 MB.
- Yahoo: 25 MB.
- Zoho: 12MB, but each file can only be 10 MB.
- AOL: 25 MB.
- ProtonMail: 25 MB
In general, the largest size you would want to attach to a mail message is 10MB in size as this will allow you to send to any email address. If your files are larger than 25MB in size, it is recommended to use some other type of cloud storage for the file and send a shared link to that file such as OneDrive, DropBox, GoogleDrive, etc.
Anti Virus/Anti-Malware
If you have an anti virus or anti malware program running on your computer it may also cause delays to emails as it has to scan each attachment for viruses or malware. If the scan delays the process enough, it can cause the sending to fail or potentially need to try a couple of times before it clears. Reducing the number of attachments can help the process speed up.
Junk/Spam Filtering
If there is no bounce back indicating there is a delay in delivery of the message, but the recipient has not received the email in 5 days; have them check their spam/junk folders, it is possible the message has been delivered, but content of the message may have been determined to have spam-like qualities. Because the process of determining spam is a mathematical calculation, even messages that are not spam can be flagged and sent to the Spam/Junk folder.
Mail Processing
Similar to the process that a traditional letter will go through there are many little stops along the way to ensure it is going to the correct location and that the email is as safe as can be determined. Looking up the address it is going to be going towards, routing it to the correct destination server. Depending on the route it takes; there could be delays or issues with one of the route's servers delaying the hand off of the message. Each hop the message takes there could be an issue with that server; or it may get held up in the spam filter on the recipient server's end - should this be the case you will receive a bounce back message indicating why it could not be delivered.