SMTP becomes slowly when sending big attachments by email

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USG_User
USG_User Posts: 369  Master Member
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edited April 2021 in Security

When sending big attachments (>10MB), the TCP connection becomes slower and slower and it could take up to hours until such a mail has finally been sent. But it neither depends on our mail server nor on our email provider. Both things have been checked. At the end only a limitation of the attachment sizeto 5 MB succeeded.

Until now we don't know who is throttling our SMTP speed.

In our USG110 we have the UTM profile "Anti-Virus" activated for any incoming or ourgoing mail transfer over all well-known mail ports, irrespective whether the connection is encrypted (SSL with STARTTLS) or not.

Could that be the reason for mail throttling when sending big attachments? Any experiences in this regard are welcome.

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  • Zyxel_Cooldia
    Zyxel_Cooldia Posts: 1,450  Zyxel Employee
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    Hi @USG_User

    What is your firmware version? 

    Can you post the CLI below when sending big attachments(>10MB). I would like to check the AS status at that time.


    Check Anti-Spam status:

    Router# show anti-spam statistics summary

    Router# show anti-spam mail-session status

    Router# show anti-spam mail-scan statistics

    Router# show anti-spam mail-session historical-high

    Check version:

    Router# show version

  • USG_User
    USG_User Posts: 369  Master Member
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    Anti-Spam is not activated for our mail security policy rule since all mails will be transmitted/received SSL(TLS) encrypted. Spam filtering will be carried out subsequently at our mail server after decryption.

    Our USG110 FW version is 4.33(AAPH.0). The current WK17 hotfix has not been applied yet.

    At the moment our production e-mail system is working fine again after limiting the attachment size to 5 MB (base64 coded, means about 3.5 MB netto file sizes). At the moment I cannot risk to deactivate the attachment limitation since we've got a lot of time-critical mails to be sent. Maybe I could give it a try on weekend.

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