diff --git a/FAQ.md b/FAQ.md
index 9a293abd11..6a133c828a 100644
--- a/FAQ.md
+++ b/FAQ.md
@@ -30,7 +30,7 @@ For authorizing:
 
 * Gmail / G suite, see [question 6](#user-content-faq6)
 * Outlook / Live / Hotmail, see [question 14](#user-content-faq14)
-* Office365, see [question 14](#user-content-faq156)
+* Office 365, see [question 14](#user-content-faq156)
 * Microsoft Exchange, see [question 8](#user-content-faq8)
 * Yahoo, AOL and Sky, see [question 88](#user-content-faq88)
 * Apple iCloud, see [question 148](#user-content-faq148)
@@ -278,7 +278,7 @@ Fonts, sizes, colors, etc should be material design whenever possible.
 * [(153) Why does permanently deleting Gmail message not work?](#user-content-faq153)
 * [~~(154) Can you add favicons as contact photos?~~](#user-content-faq154)
 * [(155) What is a winmail.dat file?](#user-content-faq155)
-* [(156) How can I set up an Office365 account?](#user-content-faq156)
+* [(156) How can I set up an Office 365 account?](#user-content-faq156)
 * [(157) How can I set up an Free.fr account?](#user-content-faq157)
 * [(158) Which camera / audio recorder do you recommend?](#user-content-faq158)
 * [(159) What are Disconnect's tracker protection lists?](#user-content-faq159)
@@ -814,7 +814,7 @@ See [here](https://support.microsoft.com/en-us/help/12409/microsoft-account-app-
 
 See [here](https://support.office.com/en-us/article/pop-imap-and-smtp-settings-for-outlook-com-d088b986-291d-42b8-9564-9c414e2aa040) for Microsoft's instructions.
 
-For setting up an Office365 account, please see [this FAQ](#user-content-faq156).
+For setting up an Office 365 account, please see [this FAQ](#user-content-faq156).
 
 <br />
 
@@ -1003,6 +1003,16 @@ If you are using a [VPN](https://en.wikipedia.org/wiki/Virtual_private_network),
 the VPN provider might block the connection because it is too aggressively trying to prevent spam.
 Note that [Google Fi](https://fi.google.com/) is using a VPN too.
 
+**Send errors**
+
+SMTP servers can reject messages for [a variety of reasons](https://en.wikipedia.org/wiki/List_of_SMTP_server_return_codes).
+Too large messages and triggering the spam filter of an email server are the most common reasons.
+
+* The attachment size limit for Gmail [is 25 MB](https://support.google.com/mail/answer/6584)
+* The attachment size limit for Outlook and Office 365 [is 20 MB](https://support.microsoft.com/en-us/help/2813269/attachment-size-exceeds-the-allowable-limit-error-when-you-add-a-large)
+* The attachment size limit for Yahoo [is 25 MB](https://help.yahoo.com/kb/SLN5673.html)
+* *554 5.7.1 Service unavailable; Client host xxx.xxx.xxx.xxx blocked*, please [see here](https://docs.gandi.net/en/gandimail/faq/error_types/554_5_7_1_service_unavailable.html)
+
 **Gmail errors**
 
 The authorization of Gmail accounts setup with the quick wizard needs to be periodically refreshed
@@ -2472,7 +2482,7 @@ because Google requires a [yearly security audit](https://support.google.com/clo
 
 OAuth for Yandex is supported via the quick setup wizard.
 
-OAuth for Office365 accounts is supported, but Microsoft does not offer OAuth for Outlook, Live and Hotmail accounts (yet?).
+OAuth for Office 365 accounts is supported, but Microsoft does not offer OAuth for Outlook, Live and Hotmail accounts (yet?).
 
 OAuth access for Yahoo was requested, but Yahoo never responded to the request.
 OAuth for AOL [was deactivated](https://www.programmableweb.com/api/aol-open-auth) by AOL.
@@ -3125,9 +3135,9 @@ You can view it with for example the Android app [Letter Opener](https://play.go
 <br />
 
 <a name="faq156"></a>
-**(156) How can I set up an Office365 account?**
+**(156) How can I set up an Office 365 account?**
 
-An Office365 account can be set up via the quick setup wizard and selecting *Office365 (OAuth)*.
+An Office 365 account can be set up via the quick setup wizard and selecting *Office 365 (OAuth)*.
 
 If the wizard ends with *AUTHENTICATE failed*, IMAP and/or SMTP might be disabled for the account.
 In this case you should ask the administrator to enable IMAP and SMTP.